resnet.py

# Copyright (c) OpenMMLab. All rights reserved.
import logging
from typing import Optional, Sequence, Tuple, Union

import torch.nn as nn
import torch.utils.checkpoint as cp
from torch import Tensor

from .utils import constant_init, kaiming_init


def conv3x3(in_planes: int,
            out_planes: int,
            stride: int = 1,
            dilation: int = 1):
    """3x3 convolution with padding."""
    return nn.Conv2d(
        in_planes,
        out_planes,
        kernel_size=3,
        stride=stride,
        padding=dilation,
        dilation=dilation,
        bias=False)


class BasicBlock(nn.Module):
    expansion = 1

    def __init__(self,
                 inplanes: int,
                 planes: int,
                 stride: int = 1,
                 dilation: int = 1,
                 downsample: Optional[nn.Module] = None,
                 style: str = 'pytorch',
                 with_cp: bool = False):
        super().__init__()
        assert style in ['pytorch', 'caffe']
        self.conv1 = conv3x3(inplanes, planes, stride, dilation)
        self.bn1 = nn.BatchNorm2d(planes)
        self.relu = nn.ReLU(inplace=True)
        self.conv2 = conv3x3(planes, planes)
        self.bn2 = nn.BatchNorm2d(planes)
        self.downsample = downsample
        self.stride = stride
        self.dilation = dilation
        assert not with_cp

    def forward(self, x: Tensor) -> Tensor:
        residual = x

        out = self.conv1(x)
        out = self.bn1(out)
        out = self.relu(out)

        out = self.conv2(out)
        out = self.bn2(out)

        if self.downsample is not None:
            residual = self.downsample(x)

        out += residual
        out = self.relu(out)

        return out


class Bottleneck(nn.Module):
    expansion = 4

    def __init__(self,
                 inplanes: int,
                 planes: int,
                 stride: int = 1,
                 dilation: int = 1,
                 downsample: Optional[nn.Module] = None,
                 style: str = 'pytorch',
                 with_cp: bool = False):
        """Bottleneck block.

        If style is "pytorch", the stride-two layer is the 3x3 conv layer, if
        it is "caffe", the stride-two layer is the first 1x1 conv layer.
        """
        super().__init__()
        assert style in ['pytorch', 'caffe']
        if style == 'pytorch':
            conv1_stride = 1
            conv2_stride = stride
        else:
            conv1_stride = stride
            conv2_stride = 1
        self.conv1 = nn.Conv2d(
            inplanes, planes, kernel_size=1, stride=conv1_stride, bias=False)
        self.conv2 = nn.Conv2d(
            planes,
            planes,
            kernel_size=3,
            stride=conv2_stride,
            padding=dilation,
            dilation=dilation,
            bias=False)

        self.bn1 = nn.BatchNorm2d(planes)
        self.bn2 = nn.BatchNorm2d(planes)
        self.conv3 = nn.Conv2d(
            planes, planes * self.expansion, kernel_size=1, bias=False)
        self.bn3 = nn.BatchNorm2d(planes * self.expansion)
        self.relu = nn.ReLU(inplace=True)
        self.downsample = downsample
        self.stride = stride
        self.dilation = dilation
        self.with_cp = with_cp

    def forward(self, x: Tensor) -> Tensor:

        def _inner_forward(x):
            residual = x

            out = self.conv1(x)
            out = self.bn1(out)
            out = self.relu(out)

            out = self.conv2(out)
            out = self.bn2(out)
            out = self.relu(out)

            out = self.conv3(out)
            out = self.bn3(out)

            if self.downsample is not None:
                residual = self.downsample(x)

            out += residual

            return out

        if self.with_cp and x.requires_grad:
            out = cp.checkpoint(_inner_forward, x)
        else:
            out = _inner_forward(x)

        out = self.relu(out)

        return out


def make_res_layer(block: nn.Module,
                   inplanes: int,
                   planes: int,
                   blocks: int,
                   stride: int = 1,
                   dilation: int = 1,
                   style: str = 'pytorch',
                   with_cp: bool = False) -> nn.Module:
    downsample = None
    if stride != 1 or inplanes != planes * block.expansion:
        downsample = nn.Sequential(
            nn.Conv2d(
                inplanes,
                planes * block.expansion,
                kernel_size=1,
                stride=stride,
                bias=False),
            nn.BatchNorm2d(planes * block.expansion),
        )

    layers = []
    layers.append(
        block(
            inplanes,
            planes,
            stride,
            dilation,
            downsample,
            style=style,
            with_cp=with_cp))
    inplanes = planes * block.expansion
    for _ in range(1, blocks):
        layers.append(
            block(inplanes, planes, 1, dilation, style=style, with_cp=with_cp))

    return nn.Sequential(*layers)


class ResNet(nn.Module):
    """ResNet backbone.

    Args:
        depth (int): Depth of resnet, from {18, 34, 50, 101, 152}.
        num_stages (int): Resnet stages, normally 4.
        strides (Sequence[int]): Strides of the first block of each stage.
        dilations (Sequence[int]): Dilation of each stage.
        out_indices (Sequence[int]): Output from which stages.
        style (str): `pytorch` or `caffe`. If set to "pytorch", the stride-two
            layer is the 3x3 conv layer, otherwise the stride-two layer is
            the first 1x1 conv layer.
        frozen_stages (int): Stages to be frozen (all param fixed). -1 means
            not freezing any parameters.
        bn_eval (bool): Whether to set BN layers as eval mode, namely, freeze
            running stats (mean and var).
        bn_frozen (bool): Whether to freeze weight and bias of BN layers.
        with_cp (bool): Use checkpoint or not. Using checkpoint will save some
            memory while slowing down the training speed.
    """

    arch_settings = {
        18: (BasicBlock, (2, 2, 2, 2)),
        34: (BasicBlock, (3, 4, 6, 3)),
        50: (Bottleneck, (3, 4, 6, 3)),
        101: (Bottleneck, (3, 4, 23, 3)),
        152: (Bottleneck, (3, 8, 36, 3))
    }

    def __init__(self,
                 depth: int,
                 num_stages: int = 4,
                 strides: Sequence[int] = (1, 2, 2, 2),
                 dilations: Sequence[int] = (1, 1, 1, 1),
                 out_indices: Sequence[int] = (0, 1, 2, 3),
                 style: str = 'pytorch',
                 frozen_stages: int = -1,
                 bn_eval: bool = True,
                 bn_frozen: bool = False,
                 with_cp: bool = False):
        super().__init__()
        if depth not in self.arch_settings:
            raise KeyError(f'invalid depth {depth} for resnet')
        assert num_stages >= 1 and num_stages <= 4
        block, stage_blocks = self.arch_settings[depth]
        stage_blocks = stage_blocks[:num_stages]  # type: ignore
        assert len(strides) == len(dilations) == num_stages
        assert max(out_indices) < num_stages

        self.out_indices = out_indices
        self.style = style
        self.frozen_stages = frozen_stages
        self.bn_eval = bn_eval
        self.bn_frozen = bn_frozen
        self.with_cp = with_cp

        self.inplanes: int = 64
        self.conv1 = nn.Conv2d(
            3, 64, kernel_size=7, stride=2, padding=3, bias=False)
        self.bn1 = nn.BatchNorm2d(64)
        self.relu = nn.ReLU(inplace=True)
        self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)

        self.res_layers = []
        for i, num_blocks in enumerate(stage_blocks):
            stride = strides[i]
            dilation = dilations[i]
            planes = 64 * 2**i
            res_layer = make_res_layer(
                block,
                self.inplanes,
                planes,
                num_blocks,
                stride=stride,
                dilation=dilation,
                style=self.style,
                with_cp=with_cp)
            self.inplanes = planes * block.expansion  # type: ignore
            layer_name = f'layer{i + 1}'
            self.add_module(layer_name, res_layer)
            self.res_layers.append(layer_name)

        self.feat_dim = block.expansion * 64 * 2**(  # type: ignore
            len(stage_blocks) - 1)

    def init_weights(self, pretrained: Optional[str] = None) -> None:
        if isinstance(pretrained, str):
            logger = logging.getLogger()
            from ..runner import load_checkpoint
            load_checkpoint(self, pretrained, strict=False, logger=logger)
        elif pretrained is None:
            for m in self.modules():
                if isinstance(m, nn.Conv2d):
                    kaiming_init(m)
                elif isinstance(m, nn.BatchNorm2d):
                    constant_init(m, 1)
        else:
            raise TypeError('pretrained must be a str or None')

    def forward(self, x: Tensor) -> Union[Tensor, Tuple[Tensor]]:
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)
        outs = []
        for i, layer_name in enumerate(self.res_layers):
            res_layer = getattr(self, layer_name)
            x = res_layer(x)
            if i in self.out_indices:
                outs.append(x)
        if len(outs) == 1:
            return outs[0]
        else:
            return tuple(outs)

    def train(self, mode: bool = True) -> None:
        super().train(mode)
        if self.bn_eval:
            for m in self.modules():
                if isinstance(m, nn.BatchNorm2d):
                    m.eval()
                    if self.bn_frozen:
                        for params in m.parameters():
                            params.requires_grad = False
        if mode and self.frozen_stages >= 0:
            for param in self.conv1.parameters():
                param.requires_grad = False
            for param in self.bn1.parameters():
                param.requires_grad = False
            self.bn1.eval()
            self.bn1.weight.requires_grad = False
            self.bn1.bias.requires_grad = False
            for i in range(1, self.frozen_stages + 1):
                mod = getattr(self, f'layer{i}')
                mod.eval()
                for param in mod.parameters():
                    param.requires_grad = False