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497 lines
20 KiB
497 lines
20 KiB
""" Nested Transformer (NesT) in PyTorch
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A PyTorch implement of Aggregating Nested Transformers as described in:
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'Aggregating Nested Transformers'
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- https://arxiv.org/abs/2105.12723
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The official Jax code is released and available at https://github.com/google-research/nested-transformer. The weights
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have been converted with convert/convert_nest_flax.py
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Acknowledgments:
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* The paper authors for sharing their research, code, and model weights
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* Ross Wightman's existing code off which I based this
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Copyright 2021 Alexander Soare
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"""
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import collections.abc
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from functools import partial
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import math
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import logging
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import numpy as np
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import torch
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from torch import nn
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import torch.nn.functional as F
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from timm.data import IMAGENET_DEFAULT_MEAN, IMAGENET_DEFAULT_STD
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from .layers import PatchEmbed, Mlp, DropPath, create_classifier, trunc_normal_
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from .layers.helpers import to_ntuple
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from .layers.create_conv2d import create_conv2d
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from .layers.pool2d_same import create_pool2d
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from .vision_transformer import Block
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from .registry import register_model
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from .helpers import build_model_with_cfg, named_apply
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from .vision_transformer import resize_pos_embed
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_logger = logging.getLogger(__name__)
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def _cfg(url='', **kwargs):
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return {
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'url': url,
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'num_classes': 1000, 'input_size': (3, 224, 224), 'pool_size': [14, 14],
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'crop_pct': .875, 'interpolation': 'bicubic', 'fixed_input_size': True,
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'mean': IMAGENET_DEFAULT_MEAN, 'std': IMAGENET_DEFAULT_STD,
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'first_conv': 'patch_embed.proj', 'classifier': 'head',
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**kwargs
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}
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default_cfgs = {
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# (weights from official Google JAX impl)
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'nest_base': _cfg(),
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'nest_small': _cfg(),
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'nest_tiny': _cfg(),
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'jx_nest_base': _cfg(url='https://www.todo-this-is-a-placeholder.com/jx_nest_base.pth'), # TODO
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'jx_nest_small': _cfg(url='https://www.todo-this-is-a-placeholder.com/jx_nest_small.pth'), # TODO
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'jx_nest_tiny': _cfg(url='https://www.todo-this-is-a-placeholder.com/jx_nest_tiny.pth'), # TODO
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}
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class Attention(nn.Module):
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"""
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This is much like `.vision_transformer.Attention` but uses *localised* self attention by accepting an input with
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an extra "image block" dim
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"""
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def __init__(self, dim, num_heads=8, qkv_bias=False, attn_drop=0., proj_drop=0.):
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super().__init__()
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self.num_heads = num_heads
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head_dim = dim // num_heads
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self.scale = head_dim ** -0.5
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self.qkv = nn.Linear(dim, 3*dim, bias=qkv_bias)
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self.attn_drop = nn.Dropout(attn_drop)
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self.proj = nn.Linear(dim, dim)
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self.proj_drop = nn.Dropout(proj_drop)
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def forward(self, x):
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"""
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x is shape: B (batch_size), T (image blocks), N (seq length per image block), C (embed dim)
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"""
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B, T, N, C = x.shape
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# result of next line is (qkv, B, num (H)eads, T, N, (C')hannels per head)
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qkv = self.qkv(x).reshape(B, T, N, 3, self.num_heads, C // self.num_heads).permute(3, 0, 4, 1, 2, 5)
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q, k, v = qkv[0], qkv[1], qkv[2] # make torchscript happy (cannot use tensor as tuple)
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attn = (q @ k.transpose(-2, -1)) * self.scale # (B, H, T, N, N)
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attn = attn.softmax(dim=-1)
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attn = self.attn_drop(attn)
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# (B, H, T, N, C'), permute -> (B, T, N, C', H)
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x = (attn @ v).permute(0, 2, 3, 4, 1).reshape(B, T, N, C)
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x = self.proj(x)
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x = self.proj_drop(x)
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return x # (B, T, N, C)
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class TransformerLayer(Block):
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"""
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This is much like `.vision_transformer.Block` but:
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- Called TransformerLayer here to allow for "block" as defined in the paper ("non-overlapping image blocks")
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- Uses modified Attention layer that handles the "block" dimension
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"""
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def __init__(self, dim, num_heads, mlp_ratio=4., qkv_bias=False, drop=0., attn_drop=0., drop_path=0.,
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act_layer=nn.GELU, norm_layer=nn.LayerNorm):
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super().__init__(dim, num_heads, mlp_ratio=4., qkv_bias=False, drop=0., attn_drop=0., drop_path=0.,
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act_layer=nn.GELU, norm_layer=nn.LayerNorm)
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self.norm1 = norm_layer(dim)
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self.attn = Attention(dim, num_heads=num_heads, qkv_bias=qkv_bias, attn_drop=attn_drop, proj_drop=drop)
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self.drop_path = DropPath(drop_path) if drop_path > 0. else nn.Identity()
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self.norm2 = norm_layer(dim)
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mlp_hidden_dim = int(dim * mlp_ratio)
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self.mlp = Mlp(in_features=dim, hidden_features=mlp_hidden_dim, act_layer=act_layer, drop=drop)
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def forward(self, x):
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y = self.norm1(x)
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x = x + self.drop_path(self.attn(y))
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x = x + self.drop_path(self.mlp(self.norm2(x)))
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return x
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class BlockAggregation(nn.Module):
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def __init__(self, in_channels, out_channels, norm_layer, pad_type=''):
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super().__init__()
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self.conv = create_conv2d(in_channels, out_channels, kernel_size=3, padding=pad_type, bias=True)
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self.norm = norm_layer(out_channels)
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self.pool = create_pool2d('max', kernel_size=3, stride=2, padding=pad_type)
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def forward(self, x):
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"""
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x is expected to have shape (B, C, H, W)
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"""
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assert x.shape[-2] % 2 == 0, 'BlockAggregation requires even input spatial dims'
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assert x.shape[-1] % 2 == 0, 'BlockAggregation requires even input spatial dims'
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x = self.conv(x)
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# Layer norm done over channel dim only
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x = self.norm(x.permute(0, 2, 3, 1)).permute(0, 3, 1, 2)
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x = self.pool(x)
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return x # (B, C, H//2, W//2)
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def blockify(x, block_size: int):
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"""image to blocks
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Args:
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x (Tensor): with shape (B, H, W, C)
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block_size (int): edge length of a single square block in units of H, W
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"""
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B, H, W, C = x.shape
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assert H % block_size == 0, '`block_size` must divide input height evenly'
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assert W % block_size == 0, '`block_size` must divide input width evenly'
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grid_height = H // block_size
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grid_width = W // block_size
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x = x.reshape(B, grid_height, block_size, grid_width, block_size, C)
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x = x.permute(0, 1, 3, 2, 4, 5)
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x = x.reshape(B, grid_height * grid_width, -1, C)
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return x # (B, T, N, C)
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def deblockify(x, block_size: int):
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"""blocks to image
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Args:
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x (Tensor): with shape (B, T, N, C) where T is number of blocks and N is sequence size per block
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block_size (int): edge length of a single square block in units of desired H, W
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"""
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B, T, _, C= x.shape
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grid_size = int(math.sqrt(T))
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x = x.reshape(B, grid_size, grid_size, block_size, block_size, C)
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x = x.permute(0, 1, 3, 2, 4, 5)
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height = width = grid_size * block_size
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x = x.reshape(B, height, width, C)
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return x # (B, H, W, C)
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class NestLevel(nn.Module):
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""" Single hierarchical level of a Nested Transformer
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"""
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def __init__(self, num_blocks, block_size, seq_length, num_heads, depth, embed_dim, mlp_ratio=4., qkv_bias=True,
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drop_rate=0., attn_drop_rate=0., drop_path_rates=[], norm_layer=None, act_layer=None):
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super().__init__()
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self.block_size = block_size
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self.pos_embed = nn.Parameter(torch.zeros(1, num_blocks, seq_length, embed_dim))
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# Transformer encoder
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if len(drop_path_rates):
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assert len(drop_path_rates) == depth, 'Must provide as many drop path rates as there are transformer layers'
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self.transformer_encoder = nn.Sequential(*[
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TransformerLayer(
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dim=embed_dim, num_heads=num_heads, mlp_ratio=mlp_ratio, qkv_bias=qkv_bias,
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drop=drop_rate, attn_drop=attn_drop_rate, drop_path=drop_path_rates[i],
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norm_layer=norm_layer, act_layer=act_layer)
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for i in range(depth)])
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def forward(self, x):
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"""
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expects x as (B, C, H, W)
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"""
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# Switch to channels last for transformer
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x = x.permute(0, 2, 3, 1) # (B, H', W', C)
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x = blockify(x, self.block_size) # (B, T, N, C')
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x = x + self.pos_embed
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x = self.transformer_encoder(x) # (B, T, N, C')
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x = deblockify(x, self.block_size) # (B, H', W', C')
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# Channel-first for block aggregation, and generally to replicate convnet feature map at each stage
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x = x.permute(0, 3, 1, 2) # (B, C, H', W')
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return x
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class Nest(nn.Module):
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""" Nested Transformer (NesT)
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A PyTorch impl of : `Aggregating Nested Transformers`
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- https://arxiv.org/abs/2105.12723
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"""
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def __init__(self, img_size=224, in_chans=3, patch_size=4, num_levels=3, embed_dims=(128, 256, 512),
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num_heads=(4, 8, 16), depths=(2, 2, 20), num_classes=1000, mlp_ratio=4., qkv_bias=True, pad_type='',
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drop_rate=0., attn_drop_rate=0., drop_path_rate=0.5, norm_layer=None, act_layer=None, weight_init='',
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global_pool='avg'):
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"""
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Args:
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img_size (int, tuple): input image size
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in_chans (int): number of input channels
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patch_size (int): patch size
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num_levels (int): number of block hierarchies (T_d in the paper)
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embed_dims (int, tuple): embedding dimensions of each level
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num_heads (int, tuple): number of attention heads for each level
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depths (int, tuple): number of transformer layers for each level
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num_classes (int): number of classes for classification head
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mlp_ratio (int): ratio of mlp hidden dim to embedding dim for MLP of transformer layers
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qkv_bias (bool): enable bias for qkv if True
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drop_rate (float): dropout rate for MLP of transformer layers, MSA final projection layer, and classifier
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attn_drop_rate (float): attention dropout rate
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drop_path_rate (float): stochastic depth rate
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norm_layer: (nn.Module): normalization layer for transformer layers
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act_layer: (nn.Module): activation layer in MLP of transformer layers
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weight_init: (str): weight init scheme
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global_pool: (str): type of pooling operation to apply to final feature map
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Notes:
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- Default values follow NesT-B from the original Jax code.
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- `embed_dims`, `num_heads`, `depths` should be ints or tuples with length `num_levels`.
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- For those following the paper, Table A1 may have errors!
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- https://github.com/google-research/nested-transformer/issues/2
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"""
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super().__init__()
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for param_name in ['embed_dims', 'num_heads', 'depths']:
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param_value = locals()[param_name]
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if isinstance(param_value, collections.abc.Sequence):
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assert len(param_value) == num_levels, f'Require `len({param_name}) == num_levels`'
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embed_dims = to_ntuple(num_levels)(embed_dims)
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num_heads = to_ntuple(num_levels)(num_heads)
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depths = to_ntuple(num_levels)(depths)
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self.num_classes = num_classes
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self.num_features = embed_dims[-1]
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norm_layer = norm_layer or partial(nn.LayerNorm, eps=1e-6)
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act_layer = act_layer or nn.GELU
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self.drop_rate = drop_rate
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self.num_levels = num_levels
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if isinstance(img_size, collections.abc.Sequence):
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assert img_size[0] == img_size[1], 'Model only handles square inputs'
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img_size = img_size[0]
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assert img_size % patch_size == 0, '`patch_size` must divide `img_size` evenly'
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self.patch_size = patch_size
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# Number of blocks at each level
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self.num_blocks = 4**(np.arange(num_levels)[::-1])
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assert (img_size // patch_size) % np.sqrt(self.num_blocks[0]) == 0, \
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'First level blocks don\'t fit evenly. Check `img_size`, `patch_size`, and `num_levels`'
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# Block edge size in units of patches
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# Hint: (img_size // patch_size) gives number of patches along edge of image. sqrt(self.num_blocks[0]) is the
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# number of blocks along edge of image
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self.block_size = int((img_size // patch_size) // np.sqrt(self.num_blocks[0]))
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# Patch embedding
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self.patch_embed = PatchEmbed(
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img_size=img_size, patch_size=patch_size, in_chans=in_chans, embed_dim=embed_dims[0])
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self.feature_info = [dict(num_chs=embed_dims[0], reduction=patch_size, module='patch_embed')]
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self.num_patches = self.patch_embed.num_patches
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self.seq_length = self.num_patches // self.num_blocks[0]
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# Build up each hierarchical level
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self.levels = nn.ModuleList([])
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self.block_aggs = nn.ModuleList([])
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drop_path_rates = [x.item() for x in torch.linspace(0, drop_path_rate, sum(depths))]
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for lix in range(self.num_levels):
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dpr = drop_path_rates[sum(depths[:lix]):sum(depths[:lix+1])]
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self.levels.append(NestLevel(
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self.num_blocks[lix], self.block_size, self.seq_length, num_heads[lix], depths[lix],
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embed_dims[lix], mlp_ratio, qkv_bias, drop_rate, attn_drop_rate, dpr, norm_layer,
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act_layer))
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self.feature_info.append(
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dict(num_chs=embed_dims[lix], reduction=self.feature_info[-1]['reduction']*2, module=f'levels.{lix}'))
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if lix < self.num_levels - 1:
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self.block_aggs.append(BlockAggregation(
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embed_dims[lix], embed_dims[lix+1], norm_layer, pad_type=pad_type))
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else:
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# Required for zipped iteration over levels and ls_block_agg together
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self.block_aggs.append(nn.Identity())
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# Final normalization layer
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self.norm = norm_layer(embed_dims[-1])
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self.feature_info.append(
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dict(num_chs=embed_dims[lix], reduction=self.feature_info[-1]['reduction'], module='norm'))
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# Classifier
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self.global_pool, self.head = create_classifier(
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self.num_features, self.num_classes, pool_type=global_pool)
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self.init_weights(weight_init)
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def init_weights(self, mode=''):
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assert mode in ('jax', 'jax_nlhb', 'nlhb', '')
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head_bias = -math.log(self.num_classes) if 'nlhb' in mode else 0.
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for level in self.levels:
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trunc_normal_(level.pos_embed, std=.02, a=-2, b=2)
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if mode.startswith('jax'):
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named_apply(partial(_init_nest_weights, head_bias=head_bias, jax_impl=True), self)
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else:
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self.apply(_init_nest_weights)
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@torch.jit.ignore
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def no_weight_decay(self):
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return {'pos_embed'}
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def get_classifier(self):
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return self.head
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def reset_classifier(self, num_classes, global_pool='avg'):
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self.num_classes = num_classes
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self.global_pool, self.head = create_classifier(
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self.num_features, self.num_classes, pool_type=global_pool)
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def forward_features(self, x):
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""" x shape (B, C, H, W)
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"""
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B, _, H, W = x.shape
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x = self.patch_embed(x)
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x = x.reshape(B, H//self.patch_size, W//self.patch_size, -1) # (B, H', W', C')
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x = x.permute(0, 3, 1, 2)
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# NOTE: TorchScript won't let us subscript module lists with integer variables, so we iterate instead
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for level, block_agg in zip(self.levels, self.block_aggs):
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x = level(x)
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x = block_agg(x)
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# Layer norm done over channel dim only
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x = self.norm(x.permute(0, 2, 3, 1)).permute(0, 3, 1, 2)
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return x
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def forward(self, x):
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""" x shape (B, C, H, W)
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"""
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x = self.forward_features(x)
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x = self.global_pool(x)
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if self.drop_rate > 0.:
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x = F.dropout(x, p=self.drop_rate, training=self.training)
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return self.head(x)
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def _init_nest_weights(module: nn.Module, name: str = '', head_bias: float = 0., jax_impl: bool = False):
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""" NesT weight initialization
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Can replicate Jax implementation. Otherwise follows vision_transformer.py
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"""
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if isinstance(module, nn.Linear):
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if name.startswith('head'):
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if jax_impl:
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trunc_normal_(module.weight, std=.02, a=-2, b=2)
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else:
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nn.init.zeros_(module.weight)
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nn.init.constant_(module.bias, head_bias)
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else:
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trunc_normal_(module.weight, std=.02, a=-2, b=2)
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if module.bias is not None:
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nn.init.zeros_(module.bias)
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elif jax_impl and isinstance(module, nn.Conv2d):
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trunc_normal_(module.weight, std=.02, a=-2, b=2)
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if module.bias is not None:
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nn.init.zeros_(module.bias)
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elif isinstance(module, (nn.LayerNorm, nn.GroupNorm, nn.BatchNorm2d)):
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nn.init.zeros_(module.bias)
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nn.init.ones_(module.weight)
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def resize_pos_embed(posemb, posemb_new):
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"""
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Rescale the grid of position embeddings when loading from state_dict
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Expected shape of position embeddings is (1, T, N, C), and considers only square images
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"""
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_logger.info('Resized position embedding: %s to %s', posemb.shape, posemb_new.shape)
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seq_length_old = posemb.shape[2]
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num_blocks_new, seq_length_new = posemb_new.shape[1:3]
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size_new = int(math.sqrt(num_blocks_new*seq_length_new))
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# First change to (1, C, H, W)
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posemb = deblockify(posemb, int(math.sqrt(seq_length_old))).permute(0, 3, 1, 2)
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posemb = F.interpolate(posemb, size=[size_new, size_new], mode='bilinear')
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# Now change to new (1, T, N, C)
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posemb = blockify(posemb.permute(0, 2, 3, 1), int(math.sqrt(seq_length_new)))
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return posemb
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|
|
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def checkpoint_filter_fn(state_dict, model):
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""" resize positional embeddings of pretrained weights """
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pos_embed_keys = [k for k in state_dict.keys() if k.startswith('pos_embed_')]
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for k in pos_embed_keys:
|
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if state_dict[k].shape != getattr(model, k).shape:
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state_dict[k] = resize_pos_embed(state_dict[k], getattr(model, k))
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return state_dict
|
|
|
|
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def _create_nest(variant, pretrained=False, default_cfg=None, **kwargs):
|
|
# if kwargs.get('features_only', None):
|
|
# raise RuntimeError('features_only not implemented for Vision Transformer models.')
|
|
|
|
default_cfg = default_cfg or default_cfgs[variant]
|
|
model = build_model_with_cfg(
|
|
Nest, variant, pretrained,
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|
default_cfg=default_cfg,
|
|
pretrained_filter_fn=checkpoint_filter_fn,
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|
feature_cfg=dict(
|
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out_indices=tuple(range(kwargs.get('num_levels', 3) + 2)), feature_cls='hook', flatten_sequential=True),
|
|
**kwargs)
|
|
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def nest_base(pretrained=False, **kwargs):
|
|
""" Nest-B @ 224x224
|
|
"""
|
|
model_kwargs = dict(
|
|
embed_dims=(128, 256, 512), num_heads=(4, 8, 16), depths=(2, 2, 20), drop_path_rate=0.5, **kwargs)
|
|
model = _create_nest('nest_base', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def nest_small(pretrained=False, **kwargs):
|
|
""" Nest-S @ 224x224
|
|
"""
|
|
model_kwargs = dict(
|
|
embed_dims=(96, 192, 384), num_heads=(3, 6, 12), depths=(2, 2, 20), drop_path_rate=0.3, **kwargs)
|
|
model = _create_nest('nest_small', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def nest_tiny(pretrained=False, **kwargs):
|
|
""" Nest-T @ 224x224
|
|
"""
|
|
model_kwargs = dict(
|
|
embed_dims=(96, 192, 384), num_heads=(3, 6, 12), depths=(2, 2, 8), drop_path_rate=0.2, **kwargs)
|
|
model = _create_nest('nest_tiny', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def jx_nest_base(pretrained=False, **kwargs):
|
|
""" Nest-B @ 224x224, Pretrained weights converted from official Jax impl.
|
|
"""
|
|
kwargs['pad_type'] = 'same'
|
|
kwargs['weight_init'] = 'jax'
|
|
model_kwargs = dict(
|
|
embed_dims=(128, 256, 512), num_heads=(4, 8, 16), depths=(2, 2, 20), drop_path_rate=0.5, **kwargs)
|
|
model = _create_nest('jx_nest_base', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def jx_nest_small(pretrained=False, **kwargs):
|
|
""" Nest-S @ 224x224, Pretrained weights converted from official Jax impl.
|
|
"""
|
|
kwargs['pad_type'] = 'same'
|
|
kwargs['weight_init'] = 'jax'
|
|
model_kwargs = dict(
|
|
embed_dims=(96, 192, 384), num_heads=(3, 6, 12), depths=(2, 2, 20), drop_path_rate=0.3, **kwargs)
|
|
model = _create_nest('jx_nest_small', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
@register_model
|
|
def jx_nest_tiny(pretrained=False, **kwargs):
|
|
""" Nest-T @ 224x224, Pretrained weights converted from official Jax impl.
|
|
"""
|
|
kwargs['pad_type'] = 'same'
|
|
kwargs['weight_init'] = 'jax'
|
|
model_kwargs = dict(
|
|
embed_dims=(96, 192, 384), num_heads=(3, 6, 12), depths=(2, 2, 8), drop_path_rate=0.2, **kwargs)
|
|
model = _create_nest('jx_nest_tiny', pretrained=pretrained, **model_kwargs)
|
|
return model
|
|
|
|
|
|
if __name__ == '__main__':
|
|
model = jx_nest_base()
|
|
model = torch.jit.script(model)
|
|
inp = torch.zeros(8, 3, 224, 224)
|
|
print(model.forward_features(inp).shape) |