pytorch-image-models/timm/models/cspnet.py

"""PyTorch CspNet

A PyTorch implementation of Cross Stage Partial Networks including:
* CSPResNet50
* CSPResNeXt50
* CSPDarkNet53
* and DarkNet53 for good measure

Based on paper `CSPNet: A New Backbone that can Enhance Learning Capability of CNN` - https://arxiv.org/abs/1911.11929

Reference impl via darknet cfg files at https://github.com/WongKinYiu/CrossStagePartialNetworks

Hacked together by / Copyright 2020 Ross Wightman
"""
import collections.abc
from dataclasses import dataclass, field, asdict
from functools import partial
from typing import Any, Callable, Dict, Optional, Tuple, Union

import torch
import torch.nn as nn
import torch.nn.functional as F

from timm.data import IMAGENET_DEFAULT_MEAN, IMAGENET_DEFAULT_STD
from .helpers import build_model_with_cfg, named_apply, MATCH_PREV_GROUP
from .layers import ClassifierHead, ConvNormAct, ConvNormActAa, DropPath, create_attn, create_act_layer, make_divisible
from .registry import register_model


__all__ = ['CspNet']  # model_registry will add each entrypoint fn to this


def _cfg(url='', **kwargs):
    return {
        'url': url,
        'num_classes': 1000, 'input_size': (3, 256, 256), 'pool_size': (8, 8),
        'crop_pct': 0.887, 'interpolation': 'bilinear',
        'mean': IMAGENET_DEFAULT_MEAN, 'std': IMAGENET_DEFAULT_STD,
        'first_conv': 'stem.conv1.conv', 'classifier': 'head.fc',
        **kwargs
    }


default_cfgs = {
    'cspresnet50': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-weights/cspresnet50_ra-d3e8d487.pth'),
    'cspresnet50d': _cfg(url=''),
    'cspresnet50w': _cfg(url=''),
    'cspresnext50': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-weights/cspresnext50_ra_224-648b4713.pth',
    ),
    'cspdarknet53': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-weights/cspdarknet53_ra_256-d05c7c21.pth'),

    'darknet17': _cfg(url=''),
    'darknet21': _cfg(url=''),
    'sedarknet21': _cfg(url=''),
    'darknet53': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-tpu-weights/darknet53_256_c2ns-3aeff817.pth',
        interpolation='bicubic', test_input_size=(3, 288, 288), test_crop_pct=1.0,
    ),
    'darknetaa53': _cfg(url=''),

    'cs3darknet_s': _cfg(
        url='', interpolation='bicubic'),
    'cs3darknet_m': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-tpu-weights/cs3darknet_m_c2ns-43f06604.pth',
        interpolation='bicubic', test_input_size=(3, 288, 288), test_crop_pct=0.95,
    ),
    'cs3darknet_l': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-tpu-weights/cs3darknet_l_c2ns-16220c5d.pth',
        interpolation='bicubic', test_input_size=(3, 288, 288), test_crop_pct=0.95),
    'cs3darknet_x': _cfg(
        url=''),

    'cs3darknet_focus_s': _cfg(
        url='', interpolation='bicubic'),
    'cs3darknet_focus_m': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-tpu-weights/cs3darknet_focus_m_c2ns-e23bed41.pth',
        interpolation='bicubic', test_input_size=(3, 288, 288), test_crop_pct=0.95),
    'cs3darknet_focus_l': _cfg(
        url='https://github.com/rwightman/pytorch-image-models/releases/download/v0.1-tpu-weights/cs3darknet_focus_l_c2ns-65ef8888.pth',
        interpolation='bicubic', test_input_size=(3, 288, 288), test_crop_pct=0.95),
    'cs3darknet_focus_x': _cfg(
        url='', interpolation='bicubic'),

    'cs3sedarknet_xdw': _cfg(
        url='', interpolation='bicubic'),
}


@dataclass
class CspStemCfg:
    out_chs: Union[int, Tuple[int, ...]] = 32
    stride: Union[int, Tuple[int, ...]] = 2
    kernel_size: int = 3
    padding: Union[int, str] = ''
    pool: Optional[str] = ''


def _pad_arg(x, n):
    # pads an argument tuple to specified n by padding with last value
    if not isinstance(x, (tuple, list)):
        x = (x,)
    curr_n = len(x)
    pad_n = n - curr_n
    if pad_n <= 0:
        return x[:n]
    return tuple(x + (x[-1],) * pad_n)


@dataclass
class CspStagesCfg:
    depth: Tuple[int, ...] = (3, 3, 5, 2)  # block depth (number of block repeats in stages)
    out_chs: Tuple[int, ...] = (128, 256, 512, 1024)  # number of output channels for blocks in stage
    stride: Union[int, Tuple[int, ...]] = 2  # stride of stage
    groups: Union[int, Tuple[int, ...]] = 1  # num kxk conv groups
    block_ratio: Union[float, Tuple[float, ...]] = 1.0
    bottle_ratio: Union[float, Tuple[float, ...]] = 1.  # bottleneck-ratio of blocks in stage
    avg_down: Union[bool, Tuple[bool, ...]] = False
    attn_layer: Optional[Union[str, Tuple[str, ...]]] = None
    stage_type: Union[str, Tuple[str]] = 'csp'  # stage type ('csp', 'cs2', 'dark')
    block_type: Union[str, Tuple[str]] = 'bottle'  # blocks type for stages ('bottle', 'dark')

    # cross-stage only
    expand_ratio: Union[float, Tuple[float, ...]] = 1.0
    cross_linear: Union[bool, Tuple[bool, ...]] = False
    down_growth: Union[bool, Tuple[bool, ...]] = False

    def __post_init__(self):
        n = len(self.depth)
        assert len(self.out_chs) == n
        self.stride = _pad_arg(self.stride, n)
        self.groups = _pad_arg(self.groups, n)
        self.block_ratio = _pad_arg(self.block_ratio, n)
        self.bottle_ratio = _pad_arg(self.bottle_ratio, n)
        self.avg_down = _pad_arg(self.avg_down, n)
        self.attn_layer = _pad_arg(self.attn_layer, n)
        self.stage_type = _pad_arg(self.stage_type, n)
        self.block_type = _pad_arg(self.block_type, n)

        self.expand_ratio = _pad_arg(self.expand_ratio, n)
        self.cross_linear = _pad_arg(self.cross_linear, n)
        self.down_growth = _pad_arg(self.down_growth, n)


@dataclass
class CspModelCfg:
    stem: CspStemCfg
    stages: CspStagesCfg
    zero_init_last: bool = True  # zero init last weight (usually bn) in residual path
    act_layer: str = 'relu'
    norm_layer: str = 'batchnorm'
    aa_layer: Optional[str] = None  # FIXME support string factory for this


def _cs3darknet_cfg(width_multiplier=1.0, depth_multiplier=1.0, avg_down=False, act_layer='silu', focus=False):
    if focus:
        stem_cfg = CspStemCfg(
            out_chs=make_divisible(64 * width_multiplier),
            kernel_size=6, stride=2, padding=2, pool='')
    else:
        stem_cfg = CspStemCfg(
            out_chs=tuple([make_divisible(c * width_multiplier) for c in (32, 64)]),
            kernel_size=3, stride=2, pool='')
    return CspModelCfg(
        stem=stem_cfg,
        stages=CspStagesCfg(
            out_chs=tuple([make_divisible(c * width_multiplier) for c in (128, 256, 512, 1024)]),
            depth=tuple([int(d * depth_multiplier) for d in (3, 6, 9, 3)]),
            stride=2,
            bottle_ratio=1.,
            block_ratio=0.5,
            avg_down=avg_down,
            stage_type='cs3',
            block_type='dark',
        ),
        act_layer=act_layer,
    )


model_cfgs = dict(
    cspresnet50=CspModelCfg(
        stem=CspStemCfg(out_chs=64, kernel_size=7, stride=4, pool='max'),
        stages=CspStagesCfg(
            depth=(3, 3, 5, 2),
            out_chs=(128, 256, 512, 1024),
            stride=(1, 2),
            expand_ratio=2.,
            bottle_ratio=0.5,
            cross_linear=True,
        ),
    ),
    cspresnet50d=CspModelCfg(
        stem=CspStemCfg(out_chs=(32, 32, 64), kernel_size=3, stride=4, pool='max'),
        stages=CspStagesCfg(
            depth=(3, 3, 5, 2),
            out_chs=(128, 256, 512, 1024),
            stride=(1,) + (2,),
            expand_ratio=2.,
            bottle_ratio=0.5,
            block_ratio=1.,
            cross_linear=True,
        )
    ),
    cspresnet50w=CspModelCfg(
        stem=CspStemCfg(out_chs=(32, 32, 64), kernel_size=3, stride=4, pool='max'),
        stages=CspStagesCfg(
            depth=(3, 3, 5, 2),
            out_chs=(256, 512, 1024, 2048),
            stride=(1,) + (2,),
            expand_ratio=1.,
            bottle_ratio=0.25,
            block_ratio=0.5,
            cross_linear=True,
        )
    ),
    cspresnext50=CspModelCfg(
        stem=CspStemCfg(out_chs=64, kernel_size=7, stride=4, pool='max'),
        stages=CspStagesCfg(
            depth=(3, 3, 5, 2),
            out_chs=(256, 512, 1024, 2048),
            stride=(1,) + (2,),
            groups=32,
            expand_ratio=1.,
            bottle_ratio=1.,
            block_ratio=0.5,
            cross_linear=True,
        )
    ),
    cspdarknet53=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1, 2, 8, 8, 4),
            out_chs=(64, 128, 256, 512, 1024),
            stride=2,
            expand_ratio=(2.,) + (1.,),
            bottle_ratio=(0.5,) + (1.,),
            block_ratio=(1.,) + (0.5,),
            down_growth=True,
            block_type='dark',
        ),
        act_layer='leaky_relu',
    ),
    darknet17=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1,) * 5,
            out_chs=(64, 128, 256, 512, 1024),
            stride=(2,),
            bottle_ratio=(0.5,),
            block_ratio=(1.,),
            stage_type='dark',
            block_type='dark',
        ),
        act_layer='leaky_relu',
    ),
    darknet21=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1, 1, 1, 2, 2),
            out_chs=(64, 128, 256, 512, 1024),
            stride=(2,),
            bottle_ratio=(0.5,),
            block_ratio=(1.,),
            stage_type='dark',
            block_type='dark',

        ),
        act_layer='leaky_relu',
    ),
    sedarknet21=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1, 1, 1, 2, 2),
            out_chs=(64, 128, 256, 512, 1024),
            stride=2,
            bottle_ratio=0.5,
            block_ratio=1.,
            attn_layer='se',
            stage_type='dark',
            block_type='dark',

        ),
        act_layer='leaky_relu',
    ),
    darknet53=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1, 2, 8, 8, 4),
            out_chs=(64, 128, 256, 512, 1024),
            stride=2,
            bottle_ratio=0.5,
            block_ratio=1.,
            stage_type='dark',
            block_type='dark',
        ),
        act_layer='leaky_relu',
    ),
    darknetaa53=CspModelCfg(
        stem=CspStemCfg(out_chs=32, kernel_size=3, stride=1, pool=''),
        stages=CspStagesCfg(
            depth=(1, 2, 8, 8, 4),
            out_chs=(64, 128, 256, 512, 1024),
            stride=2,
            bottle_ratio=0.5,
            block_ratio=1.,
            avg_down=True,
            stage_type='dark',
            block_type='dark',
        ),
        act_layer='leaky_relu',
    ),

    cs3darknet_s=_cs3darknet_cfg(width_multiplier=0.5, depth_multiplier=0.5),
    cs3darknet_m=_cs3darknet_cfg(width_multiplier=0.75, depth_multiplier=0.67),
    cs3darknet_l=_cs3darknet_cfg(),
    cs3darknet_x=_cs3darknet_cfg(width_multiplier=1.25, depth_multiplier=1.33),

    cs3darknet_focus_s=_cs3darknet_cfg(width_multiplier=0.5, depth_multiplier=0.5, focus=True),
    cs3darknet_focus_m=_cs3darknet_cfg(width_multiplier=0.75, depth_multiplier=0.67, focus=True),
    cs3darknet_focus_l=_cs3darknet_cfg(focus=True),
    cs3darknet_focus_x=_cs3darknet_cfg(width_multiplier=1.25, depth_multiplier=1.33, focus=True),

    cs3sedarknet_xdw=CspModelCfg(
        stem=CspStemCfg(out_chs=(32, 64), kernel_size=3, stride=2, pool=''),
        stages=CspStagesCfg(
            depth=(3, 6, 12, 4),
            out_chs=(256, 512, 1024, 2048),
            stride=2,
            groups=(1, 1, 256, 512),
            bottle_ratio=0.5,
            block_ratio=0.5,
            attn_layer='se',
        ),
    ),
)


class BottleneckBlock(nn.Module):
    """ ResNe(X)t Bottleneck Block
    """

    def __init__(
            self,
            in_chs,
            out_chs,
            dilation=1,
            bottle_ratio=0.25,
            groups=1,
            act_layer=nn.ReLU,
            norm_layer=nn.BatchNorm2d,
            attn_last=False,
            attn_layer=None,
            aa_layer=None,
            drop_block=None,
            drop_path=0.
    ):
        super(BottleneckBlock, self).__init__()
        mid_chs = int(round(out_chs * bottle_ratio))
        ckwargs = dict(act_layer=act_layer, norm_layer=norm_layer)

        self.conv1 = ConvNormAct(in_chs, mid_chs, kernel_size=1, **ckwargs)
        self.conv2 = ConvNormActAa(
            mid_chs, mid_chs, kernel_size=3, dilation=dilation, groups=groups,
            aa_layer=aa_layer, drop_layer=drop_block, **ckwargs)
        self.attn2 = create_attn(attn_layer, channels=mid_chs) if not attn_last else None
        self.conv3 = ConvNormAct(mid_chs, out_chs, kernel_size=1, apply_act=False, **ckwargs)
        self.attn3 = create_attn(attn_layer, channels=out_chs) if attn_last else None
        self.drop_path = DropPath(drop_path) if drop_path else nn.Identity()
        self.act3 = create_act_layer(act_layer)

    def zero_init_last(self):
        nn.init.zeros_(self.conv3.bn.weight)

    def forward(self, x):
        shortcut = x
        x = self.conv1(x)
        x = self.conv2(x)
        if self.attn2 is not None:
            x = self.attn2(x)
        x = self.conv3(x)
        if self.attn3 is not None:
            x = self.attn3(x)
        x = self.drop_path(x) + shortcut
        # FIXME partial shortcut needed if first block handled as per original, not used for my current impl
        #x[:, :shortcut.size(1)] += shortcut
        x = self.act3(x)
        return x


class DarkBlock(nn.Module):
    """ DarkNet Block
    """

    def __init__(
            self,
            in_chs,
            out_chs,
            dilation=1,
            bottle_ratio=0.5,
            groups=1,
            act_layer=nn.ReLU,
            norm_layer=nn.BatchNorm2d,
            attn_layer=None,
            aa_layer=None,
            drop_block=None,
            drop_path=0.
    ):
        super(DarkBlock, self).__init__()
        mid_chs = int(round(out_chs * bottle_ratio))
        ckwargs = dict(act_layer=act_layer, norm_layer=norm_layer)
        self.conv1 = ConvNormAct(in_chs, mid_chs, kernel_size=1, **ckwargs)
        self.conv2 = ConvNormActAa(
            mid_chs, out_chs, kernel_size=3, dilation=dilation, groups=groups,
            aa_layer=aa_layer, drop_layer=drop_block, **ckwargs)
        self.attn = create_attn(attn_layer, channels=out_chs, act_layer=act_layer)
        self.drop_path = DropPath(drop_path) if drop_path else nn.Identity()

    def zero_init_last(self):
        nn.init.zeros_(self.conv2.bn.weight)

    def forward(self, x):
        shortcut = x
        x = self.conv1(x)
        x = self.conv2(x)
        if self.attn is not None:
            x = self.attn(x)
        x = self.drop_path(x) + shortcut
        return x


class CrossStage(nn.Module):
    """Cross Stage."""
    def __init__(
            self,
            in_chs,
            out_chs,
            stride,
            dilation,
            depth,
            block_ratio=1.,
            bottle_ratio=1.,
            expand_ratio=1.,
            groups=1,
            first_dilation=None,
            avg_down=False,
            down_growth=False,
            cross_linear=False,
            block_dpr=None,
            block_fn=BottleneckBlock,
            **block_kwargs
    ):
        super(CrossStage, self).__init__()
        first_dilation = first_dilation or dilation
        down_chs = out_chs if down_growth else in_chs  # grow downsample channels to output channels
        self.expand_chs = exp_chs = int(round(out_chs * expand_ratio))
        block_out_chs = int(round(out_chs * block_ratio))
        conv_kwargs = dict(act_layer=block_kwargs.get('act_layer'), norm_layer=block_kwargs.get('norm_layer'))

        if stride != 1 or first_dilation != dilation:
            if avg_down:
                self.conv_down = nn.Sequential(
                    nn.AvgPool2d(2) if stride == 2 else nn.Identity(),  # FIXME dilation handling
                    ConvNormActAa(in_chs, out_chs, kernel_size=1, stride=1, groups=groups, **conv_kwargs)
                )
            else:
                self.conv_down = ConvNormActAa(
                    in_chs, down_chs, kernel_size=3, stride=stride, dilation=first_dilation, groups=groups,
                    aa_layer=block_kwargs.get('aa_layer', None), **conv_kwargs)
            prev_chs = down_chs
        else:
            self.conv_down = nn.Identity()
            prev_chs = in_chs

        # FIXME this 1x1 expansion is pushed down into the cross and block paths in the darknet cfgs. Also,
        # there is also special case for the first stage for some of the model that results in uneven split
        # across the two paths. I did it this way for simplicity for now.
        self.conv_exp = ConvNormAct(prev_chs, exp_chs, kernel_size=1, apply_act=not cross_linear, **conv_kwargs)
        prev_chs = exp_chs // 2  # output of conv_exp is always split in two

        self.blocks = nn.Sequential()
        for i in range(depth):
            self.blocks.add_module(str(i), block_fn(
                in_chs=prev_chs,
                out_chs=block_out_chs,
                dilation=dilation,
                bottle_ratio=bottle_ratio,
                groups=groups,
                drop_path=block_dpr[i] if block_dpr is not None else 0.,
                **block_kwargs
            ))
            prev_chs = block_out_chs

        # transition convs
        self.conv_transition_b = ConvNormAct(prev_chs, exp_chs // 2, kernel_size=1, **conv_kwargs)
        self.conv_transition = ConvNormAct(exp_chs, out_chs, kernel_size=1, **conv_kwargs)

    def forward(self, x):
        x = self.conv_down(x)
        x = self.conv_exp(x)
        xs, xb = x.split(self.expand_chs // 2, dim=1)
        xb = self.blocks(xb)
        xb = self.conv_transition_b(xb).contiguous()
        out = self.conv_transition(torch.cat([xs, xb], dim=1))
        return out


class CrossStage3(nn.Module):
    """Cross Stage 3.
    Similar to CrossStage, but with only one transition conv for the output.
    """
    def __init__(
            self,
            in_chs,
            out_chs,
            stride,
            dilation,
            depth,
            block_ratio=1.,
            bottle_ratio=1.,
            expand_ratio=1.,
            groups=1,
            first_dilation=None,
            avg_down=False,
            down_growth=False,
            cross_linear=False,
            block_dpr=None,
            block_fn=BottleneckBlock,
            **block_kwargs
    ):
        super(CrossStage3, self).__init__()
        first_dilation = first_dilation or dilation
        down_chs = out_chs if down_growth else in_chs  # grow downsample channels to output channels
        self.expand_chs = exp_chs = int(round(out_chs * expand_ratio))
        block_out_chs = int(round(out_chs * block_ratio))
        conv_kwargs = dict(act_layer=block_kwargs.get('act_layer'), norm_layer=block_kwargs.get('norm_layer'))

        if stride != 1 or first_dilation != dilation:
            if avg_down:
                self.conv_down = nn.Sequential(
                    nn.AvgPool2d(2) if stride == 2 else nn.Identity(),  # FIXME dilation handling
                    ConvNormActAa(in_chs, out_chs, kernel_size=1, stride=1, groups=groups, **conv_kwargs)
                )
            else:
                self.conv_down = ConvNormActAa(
                    in_chs, down_chs, kernel_size=3, stride=stride, dilation=first_dilation, groups=groups,
                    aa_layer=block_kwargs.get('aa_layer', None), **conv_kwargs)
            prev_chs = down_chs
        else:
            self.conv_down = None
            prev_chs = in_chs

        # expansion conv
        self.conv_exp = ConvNormAct(prev_chs, exp_chs, kernel_size=1, apply_act=not cross_linear, **conv_kwargs)
        prev_chs = exp_chs // 2  # expanded output is split in 2 for blocks and cross stage

        self.blocks = nn.Sequential()
        for i in range(depth):
            self.blocks.add_module(str(i), block_fn(
                in_chs=prev_chs,
                out_chs=block_out_chs,
                dilation=dilation,
                bottle_ratio=bottle_ratio,
                groups=groups,
                drop_path=block_dpr[i] if block_dpr is not None else 0.,
                **block_kwargs
            ))
            prev_chs = block_out_chs

        # transition convs
        self.conv_transition = ConvNormAct(exp_chs, out_chs, kernel_size=1, **conv_kwargs)

    def forward(self, x):
        x = self.conv_down(x)
        x = self.conv_exp(x)
        x1, x2 = x.split(self.expand_chs // 2, dim=1)
        x1 = self.blocks(x1)
        out = self.conv_transition(torch.cat([x1, x2], dim=1))
        return out


class DarkStage(nn.Module):
    """DarkNet stage."""

    def __init__(
            self,
            in_chs,
            out_chs,
            stride,
            dilation,
            depth,
            block_ratio=1.,
            bottle_ratio=1.,
            groups=1,
            first_dilation=None,
            avg_down=False,
            block_fn=BottleneckBlock,
            block_dpr=None,
            **block_kwargs
    ):
        super(DarkStage, self).__init__()
        first_dilation = first_dilation or dilation
        conv_kwargs = dict(act_layer=block_kwargs.get('act_layer'), norm_layer=block_kwargs.get('norm_layer'))

        if avg_down:
            self.conv_down = nn.Sequential(
                nn.AvgPool2d(2) if stride == 2 else nn.Identity(),   # FIXME dilation handling
                ConvNormActAa(in_chs, out_chs, kernel_size=1, stride=1, groups=groups, **conv_kwargs)
            )
        else:
            self.conv_down = ConvNormActAa(
                in_chs, out_chs, kernel_size=3, stride=stride, dilation=first_dilation, groups=groups,
                aa_layer=block_kwargs.get('aa_layer', None), **conv_kwargs)

        prev_chs = out_chs
        block_out_chs = int(round(out_chs * block_ratio))
        self.blocks = nn.Sequential()
        for i in range(depth):
            self.blocks.add_module(str(i), block_fn(
                in_chs=prev_chs,
                out_chs=block_out_chs,
                dilation=dilation,
                bottle_ratio=bottle_ratio,
                groups=groups,
                drop_path=block_dpr[i] if block_dpr is not None else 0.,
                **block_kwargs
            ))
            prev_chs = block_out_chs

    def forward(self, x):
        x = self.conv_down(x)
        x = self.blocks(x)
        return x


def create_csp_stem(
        in_chans=3,
        out_chs=32,
        kernel_size=3,
        stride=2,
        pool='',
        padding='',
        act_layer=nn.ReLU,
        norm_layer=nn.BatchNorm2d,
        aa_layer=None
):
    stem = nn.Sequential()
    feature_info = []
    if not isinstance(out_chs, (tuple, list)):
        out_chs = [out_chs]
    stem_depth = len(out_chs)
    assert stem_depth
    assert stride in (1, 2, 4)
    prev_feat = None
    prev_chs = in_chans
    last_idx = stem_depth - 1
    stem_stride = 1
    for i, chs in enumerate(out_chs):
        conv_name = f'conv{i + 1}'
        conv_stride = 2 if (i == 0 and stride > 1) or (i == last_idx and stride > 2 and not pool) else 1
        if conv_stride > 1 and prev_feat is not None:
            feature_info.append(prev_feat)
        stem.add_module(conv_name, ConvNormAct(
            prev_chs, chs, kernel_size,
            stride=conv_stride,
            padding=padding if i == 0 else '',
            act_layer=act_layer,
            norm_layer=norm_layer
        ))
        stem_stride *= conv_stride
        prev_chs = chs
        prev_feat = dict(num_chs=prev_chs, reduction=stem_stride, module='.'.join(['stem', conv_name]))
    if pool:
        assert stride > 2
        if prev_feat is not None:
            feature_info.append(prev_feat)
        if aa_layer is not None:
            stem.add_module('pool', nn.MaxPool2d(kernel_size=3, stride=1, padding=1))
            stem.add_module('aa', aa_layer(channels=prev_chs, stride=2))
            pool_name = 'aa'
        else:
            stem.add_module('pool', nn.MaxPool2d(kernel_size=3, stride=2, padding=1))
            pool_name = 'pool'
        stem_stride *= 2
        prev_feat = dict(num_chs=prev_chs, reduction=stem_stride, module='.'.join(['stem', pool_name]))
    feature_info.append(prev_feat)
    return stem, feature_info


def _get_stage_fn(stage_type: str, stage_args):
    assert stage_type in ('dark', 'csp', 'cs3')
    if stage_type == 'dark':
        stage_args.pop('expand_ratio', None)
        stage_args.pop('cross_linear', None)
        stage_args.pop('down_growth', None)
        stage_fn = DarkStage
    elif stage_type == 'csp':
        stage_fn = CrossStage
    else:
        stage_fn = CrossStage3
    return stage_fn, stage_args


def _get_block_fn(stage_type: str, stage_args):
    assert stage_type in ('dark', 'bottle')
    if stage_type == 'dark':
        return DarkBlock, stage_args
    else:
        return BottleneckBlock, stage_args


def create_csp_stages(
        cfg: CspModelCfg,
        drop_path_rate: float,
        output_stride: int,
        stem_feat: Dict[str, Any]
):
    cfg_dict = asdict(cfg.stages)
    num_stages = len(cfg.stages.depth)
    cfg_dict['block_dpr'] = [None] * num_stages if not drop_path_rate else \
        [x.tolist() for x in torch.linspace(0, drop_path_rate, sum(cfg.stages.depth)).split(cfg.stages.depth)]
    stage_args = [dict(zip(cfg_dict.keys(), values)) for values in zip(*cfg_dict.values())]
    block_kwargs = dict(
        act_layer=cfg.act_layer,
        norm_layer=cfg.norm_layer,
        aa_layer=cfg.aa_layer
    )

    dilation = 1
    net_stride = stem_feat['reduction']
    prev_chs = stem_feat['num_chs']
    prev_feat = stem_feat
    feature_info = []
    stages = []
    for stage_idx, stage_args in enumerate(stage_args):
        stage_fn, stage_args = _get_stage_fn(stage_args.pop('stage_type'), stage_args)
        block_fn, stage_args = _get_block_fn(stage_args.pop('block_type'), stage_args)
        stride = stage_args.pop('stride')
        if stride != 1 and prev_feat:
            feature_info.append(prev_feat)
        if net_stride >= output_stride and stride > 1:
            dilation *= stride
            stride = 1
        net_stride *= stride
        first_dilation = 1 if dilation in (1, 2) else 2

        stages += [stage_fn(
            prev_chs,
            **stage_args,
            stride=stride,
            first_dilation=first_dilation,
            dilation=dilation,
            block_fn=block_fn,
            **block_kwargs,
        )]
        prev_chs = stage_args['out_chs']
        prev_feat = dict(num_chs=prev_chs, reduction=net_stride, module=f'stages.{stage_idx}')

    feature_info.append(prev_feat)
    return nn.Sequential(*stages), feature_info


class CspNet(nn.Module):
    """Cross Stage Partial base model.

    Paper: `CSPNet: A New Backbone that can Enhance Learning Capability of CNN` - https://arxiv.org/abs/1911.11929
    Ref Impl: https://github.com/WongKinYiu/CrossStagePartialNetworks

    NOTE: There are differences in the way I handle the 1x1 'expansion' conv in this impl vs the
    darknet impl. I did it this way for simplicity and less special cases.
    """

    def __init__(
            self,
            cfg: CspModelCfg,
            in_chans=3,
            num_classes=1000,
            output_stride=32,
            global_pool='avg',
            drop_rate=0.,
            drop_path_rate=0.,
            zero_init_last=True
    ):
        super().__init__()
        self.num_classes = num_classes
        self.drop_rate = drop_rate
        assert output_stride in (8, 16, 32)
        layer_args = dict(
            act_layer=cfg.act_layer,
            norm_layer=cfg.norm_layer,
            aa_layer=cfg.aa_layer
        )
        self.feature_info = []

        # Construct the stem
        self.stem, stem_feat_info = create_csp_stem(in_chans, **asdict(cfg.stem), **layer_args)
        self.feature_info.extend(stem_feat_info[:-1])

        # Construct the stages
        self.stages, stage_feat_info = create_csp_stages(
            cfg,
            drop_path_rate=drop_path_rate,
            output_stride=output_stride,
            stem_feat=stem_feat_info[-1],
        )
        prev_chs = stage_feat_info[-1]['num_chs']
        self.feature_info.extend(stage_feat_info)

        # Construct the head
        self.num_features = prev_chs
        self.head = ClassifierHead(
            in_chs=prev_chs, num_classes=num_classes, pool_type=global_pool, drop_rate=drop_rate)

        named_apply(partial(_init_weights, zero_init_last=zero_init_last), self)

    @torch.jit.ignore
    def group_matcher(self, coarse=False):
        matcher = dict(
            stem=r'^stem',
            blocks=r'^stages\.(\d+)' if coarse else [
                (r'^stages\.(\d+)\.blocks\.(\d+)', None),
                (r'^stages\.(\d+)\..*transition', MATCH_PREV_GROUP),  # map to last block in stage
                (r'^stages\.(\d+)', (0,)),
            ]
        )
        return matcher

    @torch.jit.ignore
    def set_grad_checkpointing(self, enable=True):
        assert not enable, 'gradient checkpointing not supported'

    @torch.jit.ignore
    def get_classifier(self):
        return self.head.fc

    def reset_classifier(self, num_classes, global_pool='avg'):
        self.head = ClassifierHead(self.num_features, num_classes, pool_type=global_pool, drop_rate=self.drop_rate)

    def forward_features(self, x):
        x = self.stem(x)
        x = self.stages(x)
        return x

    def forward_head(self, x, pre_logits: bool = False):
        return self.head(x, pre_logits=pre_logits)

    def forward(self, x):
        x = self.forward_features(x)
        x = self.forward_head(x)
        return x


def _init_weights(module, name, zero_init_last=False):
    if isinstance(module, nn.Conv2d):
        nn.init.kaiming_normal_(module.weight, mode='fan_out', nonlinearity='relu')
        if module.bias is not None:
            nn.init.zeros_(module.bias)
    elif isinstance(module, nn.Linear):
        nn.init.normal_(module.weight, mean=0.0, std=0.01)
        if module.bias is not None:
            nn.init.zeros_(module.bias)
    elif zero_init_last and hasattr(module, 'zero_init_last'):
        module.zero_init_last()


def _create_cspnet(variant, pretrained=False, **kwargs):
    if variant.startswith('darknet') or variant.startswith('cspdarknet'):
        # NOTE: DarkNet is one of few models with stride==1 features w/ 6 out_indices [0..5]
        default_out_indices = (0, 1, 2, 3, 4, 5)
    else:
        default_out_indices = (0, 1, 2, 3, 4)
    out_indices = kwargs.pop('out_indices', default_out_indices)
    return build_model_with_cfg(
        CspNet, variant, pretrained,
        model_cfg=model_cfgs[variant],
        feature_cfg=dict(flatten_sequential=True, out_indices=out_indices),
        **kwargs)


@register_model
def cspresnet50(pretrained=False, **kwargs):
    return _create_cspnet('cspresnet50', pretrained=pretrained, **kwargs)


@register_model
def cspresnet50d(pretrained=False, **kwargs):
    return _create_cspnet('cspresnet50d', pretrained=pretrained, **kwargs)


@register_model
def cspresnet50w(pretrained=False, **kwargs):
    return _create_cspnet('cspresnet50w', pretrained=pretrained, **kwargs)


@register_model
def cspresnext50(pretrained=False, **kwargs):
    return _create_cspnet('cspresnext50', pretrained=pretrained, **kwargs)


@register_model
def cspdarknet53(pretrained=False, **kwargs):
    return _create_cspnet('cspdarknet53', pretrained=pretrained, **kwargs)


@register_model
def darknet17(pretrained=False, **kwargs):
    return _create_cspnet('darknet17', pretrained=pretrained, **kwargs)


@register_model
def darknet21(pretrained=False, **kwargs):
    return _create_cspnet('darknet21', pretrained=pretrained, **kwargs)


@register_model
def sedarknet21(pretrained=False, **kwargs):
    return _create_cspnet('sedarknet21', pretrained=pretrained, **kwargs)


@register_model
def darknet53(pretrained=False, **kwargs):
    return _create_cspnet('darknet53', pretrained=pretrained, **kwargs)


@register_model
def darknetaa53(pretrained=False, **kwargs):
    return _create_cspnet('darknetaa53', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_s(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_s', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_m(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_m', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_l(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_l', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_x(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_x', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_focus_s(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_focus_s', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_focus_m(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_focus_m', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_focus_l(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_focus_l', pretrained=pretrained, **kwargs)


@register_model
def cs3darknet_focus_x(pretrained=False, **kwargs):
    return _create_cspnet('cs3darknet_focus_x', pretrained=pretrained, **kwargs)


@register_model
def cs3sedarknet_xdw(pretrained=False, **kwargs):
    return _create_cspnet('cs3sedarknet_xdw', pretrained=pretrained, **kwargs)