backbone/model_resnet.py

import torch.nn as nn
from torch.nn import (
    Linear,
    Conv2d,
    BatchNorm1d,
    BatchNorm2d,
    ReLU,
    Dropout,
    MaxPool2d,
    Sequential,
    Module,
)


# Support: ['ResNet_50', 'ResNet_101', 'ResNet_152']


def conv3x3(in_planes, out_planes, stride=1):
    """3x3 convolution with padding"""

    return Conv2d(
        in_planes, out_planes, kernel_size=3, stride=stride, padding=1, bias=False
    )


def conv1x1(in_planes, out_planes, stride=1):
    """1x1 convolution"""

    return Conv2d(in_planes, out_planes, kernel_size=1, stride=stride, bias=False)


class BasicBlock(Module):
    expansion = 1

    def __init__(self, inplanes, planes, stride=1, downsample=None):
        super(BasicBlock, self).__init__()
        self.conv1 = conv3x3(inplanes, planes, stride)
        self.bn1 = BatchNorm2d(planes)
        self.relu = ReLU(inplace=True)
        self.conv2 = conv3x3(planes, planes)
        self.bn2 = BatchNorm2d(planes)
        self.downsample = downsample
        self.stride = stride

    def forward(self, x):
        identity = 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:
            identity = self.downsample(x)

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

        return out


class Bottleneck(Module):
    expansion = 4

    def __init__(self, inplanes, planes, stride=1, downsample=None):
        super(Bottleneck, self).__init__()
        self.conv1 = conv1x1(inplanes, planes)
        self.bn1 = BatchNorm2d(planes)
        self.conv2 = conv3x3(planes, planes, stride)
        self.bn2 = BatchNorm2d(planes)
        self.conv3 = conv1x1(planes, planes * self.expansion)
        self.bn3 = BatchNorm2d(planes * self.expansion)
        self.relu = ReLU(inplace=True)
        self.downsample = downsample
        self.stride = stride

    def forward(self, x):
        identity = 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:
            identity = self.downsample(x)

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

        return out


class ResNet(Module):
    def __init__(self, input_size, block, layers, zero_init_residual=True):
        super(ResNet, self).__init__()
        assert input_size[0] in [
            112,
            224,
        ], "input_size should be [112, 112] or [224, 224]"
        self.inplanes = 64
        self.conv1 = Conv2d(3, 64, kernel_size=7, stride=2, padding=3, bias=False)
        self.bn1 = BatchNorm2d(64)
        self.relu = ReLU(inplace=True)
        self.maxpool = MaxPool2d(kernel_size=3, stride=2, padding=1)
        self.layer1 = self._make_layer(block, 64, layers[0])
        self.layer2 = self._make_layer(block, 128, layers[1], stride=2)
        self.layer3 = self._make_layer(block, 256, layers[2], stride=2)
        self.layer4 = self._make_layer(block, 512, layers[3], stride=2)

        self.bn_o1 = BatchNorm2d(2048)
        self.dropout = Dropout()
        if input_size[0] == 112:
            self.fc = Linear(2048 * 4 * 4, 512)
        else:
            self.fc = Linear(2048 * 8 * 8, 512)
        self.bn_o2 = BatchNorm1d(512)

        for m in self.modules():
            if isinstance(m, Conv2d):
                nn.init.kaiming_normal_(m.weight, mode="fan_out", nonlinearity="relu")
            elif isinstance(m, BatchNorm2d):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)

        # Zero-initialize the last BN in each residual branch,
        # so that the residual branch starts with zeros, and each residual block behaves like an identity.
        # This improves the model by 0.2~0.3% according to https://arxiv.org/abs/1706.02677
        if zero_init_residual:
            for m in self.modules():
                if isinstance(m, Bottleneck):
                    nn.init.constant_(m.bn3.weight, 0)
                elif isinstance(m, BasicBlock):
                    nn.init.constant_(m.bn2.weight, 0)

    def _make_layer(self, block, planes, blocks, stride=1):
        downsample = None
        if stride != 1 or self.inplanes != planes * block.expansion:
            downsample = Sequential(
                conv1x1(self.inplanes, planes * block.expansion, stride),
                BatchNorm2d(planes * block.expansion),
            )

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

        return Sequential(*layers)

    def forward(self, x):
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)

        x = self.layer1(x)
        x = self.layer2(x)
        x = self.layer3(x)
        x = self.layer4(x)

        x = self.bn_o1(x)
        x = self.dropout(x)
        x = x.view(x.size(0), -1)
        x = self.fc(x)
        x = self.bn_o2(x)

        return x


def ResNet_18(input_size, **kwargs):
    """Constructs a ResNet-50 model."""
    model = ResNet(input_size, Bottleneck, [2, 2, 2, 2], **kwargs)

    return model


def ResNet_50(input_size, **kwargs):
    """Constructs a ResNet-50 model."""
    model = ResNet(input_size, Bottleneck, [3, 4, 6, 3], **kwargs)

    return model


def ResNet_101(input_size, **kwargs):
    """Constructs a ResNet-101 model."""
    model = ResNet(input_size, Bottleneck, [3, 4, 23, 3], **kwargs)

    return model


def ResNet_152(input_size, **kwargs):
    """Constructs a ResNet-152 model."""
    model = ResNet(input_size, Bottleneck, [3, 8, 36, 3], **kwargs)

    return model