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116 lines
4.4 KiB
116 lines
4.4 KiB
""" Lambda Layer
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Paper: `LambdaNetworks: Modeling Long-Range Interactions Without Attention`
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- https://arxiv.org/abs/2102.08602
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@misc{2102.08602,
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Author = {Irwan Bello},
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Title = {LambdaNetworks: Modeling Long-Range Interactions Without Attention},
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Year = {2021},
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}
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Status:
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This impl is a WIP. Code snippets in the paper were used as reference but
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good chance some details are missing/wrong.
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I've only implemented local lambda conv based pos embeddings.
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For a PyTorch impl that includes other embedding options checkout
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https://github.com/lucidrains/lambda-networks
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Hacked together by / Copyright 2021 Ross Wightman
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"""
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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 .helpers import to_2tuple
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from .weight_init import trunc_normal_
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def rel_pos_indices(size):
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size = to_2tuple(size)
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pos = torch.stack(torch.meshgrid(torch.arange(size[0]), torch.arange(size[1]))).flatten(1)
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rel_pos = pos[:, None, :] - pos[:, :, None]
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rel_pos[0] += size[0] - 1
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rel_pos[1] += size[1] - 1
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return rel_pos # 2, H * W, H * W
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class LambdaLayer(nn.Module):
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"""Lambda Layer
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Paper: `LambdaNetworks: Modeling Long-Range Interactions Without Attention`
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- https://arxiv.org/abs/2102.08602
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NOTE: intra-depth parameter 'u' is fixed at 1. It did not appear worth the complexity to add.
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"""
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def __init__(
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self,
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dim, dim_out=None, feat_size=None, stride=1, num_heads=4, dim_head=16, r=7, qkv_bias=False):
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super().__init__()
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self.dim = dim
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self.dim_out = dim_out or dim
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self.dim_k = dim_head # query depth 'k'
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self.num_heads = num_heads
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assert self.dim_out % num_heads == 0, ' should be divided by num_heads'
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self.dim_v = self.dim_out // num_heads # value depth 'v'
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self.qkv = nn.Conv2d(
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dim,
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num_heads * dim_head + dim_head + self.dim_v,
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kernel_size=1, bias=qkv_bias)
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self.norm_q = nn.BatchNorm2d(num_heads * dim_head)
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self.norm_v = nn.BatchNorm2d(self.dim_v)
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if r is not None:
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# local lambda convolution for pos
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self.conv_lambda = nn.Conv3d(1, dim_head, (r, r, 1), padding=(r // 2, r // 2, 0))
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self.pos_emb = None
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self.rel_pos_indices = None
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else:
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# relative pos embedding
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assert feat_size is not None
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feat_size = to_2tuple(feat_size)
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rel_size = [2 * s - 1 for s in feat_size]
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self.conv_lambda = None
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self.pos_emb = nn.Parameter(torch.zeros(rel_size[0], rel_size[1], self.dim_k))
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self.register_buffer('rel_pos_indices', rel_pos_indices(feat_size), persistent=False)
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self.pool = nn.AvgPool2d(2, 2) if stride == 2 else nn.Identity()
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self.reset_parameters()
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def reset_parameters(self):
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trunc_normal_(self.qkv.weight, std=self.dim ** -0.5)
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if self.conv_lambda is not None:
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trunc_normal_(self.conv_lambda.weight, std=self.dim_k ** -0.5)
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if self.pos_emb is not None:
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trunc_normal_(self.pos_emb, std=.02)
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def forward(self, x):
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B, C, H, W = x.shape
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M = H * W
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qkv = self.qkv(x)
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q, k, v = torch.split(qkv, [
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self.num_heads * self.dim_k, self.dim_k, self.dim_v], dim=1)
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q = self.norm_q(q).reshape(B, self.num_heads, self.dim_k, M).transpose(-1, -2) # B, num_heads, M, K
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v = self.norm_v(v).reshape(B, self.dim_v, M).transpose(-1, -2) # B, M, V
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k = F.softmax(k.reshape(B, self.dim_k, M), dim=-1) # B, K, M
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content_lam = k @ v # B, K, V
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content_out = q @ content_lam.unsqueeze(1) # B, num_heads, M, V
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if self.pos_emb is None:
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position_lam = self.conv_lambda(v.reshape(B, 1, H, W, self.dim_v)) # B, H, W, V, K
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position_lam = position_lam.reshape(B, 1, self.dim_k, H * W, self.dim_v).transpose(2, 3) # B, 1, M, K, V
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else:
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# FIXME relative pos embedding path not fully verified
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pos_emb = self.pos_emb[self.rel_pos_indices[0], self.rel_pos_indices[1]].expand(B, -1, -1, -1)
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position_lam = (pos_emb.transpose(-1, -2) @ v.unsqueeze(1)).unsqueeze(1) # B, 1, M, K, V
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position_out = (q.unsqueeze(-2) @ position_lam).squeeze(-2) # B, num_heads, M, V
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out = (content_out + position_out).transpose(-1, -2).reshape(B, C, H, W) # B, C (num_heads * V), H, W
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out = self.pool(out)
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return out
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