选取经典的早期Pytorch官方实现代码进行分析

https://github.com/pytorch/vision/blob/9a481d0bec2700763a799ff148fe2e083b575441/torchvision/models/resnet.py
各种ResNet网络是由BasicBlock或者bottleneck构成的，它们是构成深度残差网络的基本模块

ResNet主体

ResNet的大部分各种结构是1层conv+4个block+1层fc

class ResNet(nn.Module):
    def __init__(self, block, layers, zero_init_residual=False):
        super(ResNet, self).__init__()
        self.inplanes = 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.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.avgpool = nn.AdaptiveAvgPool2d((1, 1))
        self.fc = nn.Linear(512 * block.expansion, num_classes)
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                nn.init.kaiming_normal_(m.weight, mode='fan_out', nonlinearity='relu')
            elif isinstance(m, nn.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:
         # normly happened when stride = 2
            downsample = nn.Sequential(
                conv1x1(self.inplanes, planes * block.expansion, stride),
                nn.BatchNorm2d(planes * block.expansion),
            )
        layers = []
        layers.append(block(self.inplanes, planes, stride, downsample))
        self.inplanes = planes * block.expansion
        for _ in range(1, blocks):  
        # only the first block need downsample thus there is no downsample and stride = 2
            layers.append(block(self.inplanes, planes))
        return nn.Sequential(*layers)
    def forward(self, x):
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu(x)
        x = self.maxpool(x)
        c2 = self.layer1(x)
        c3 = self.layer2(c2)
        c4 = self.layer3(c3)
        c5 = self.layer4(c4)
        x = self.avgpool(x)
        x = x.view(x.size(0), -1)
        x = self.fc(x)
        return c5

需要注意的是最后的avgpool是全局的平均池化

BasicBlock

class BasicBlock(nn.Module):
    expansion = 1
    def __init__(self, inplanes, planes, stride=1, downsample=None):
    	# here planes names channel number
        super(BasicBlock, self).__init__()
        self.conv1 = conv3x3(inplanes, planes, stride)
        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
    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

图1. BasicBlock结构图¹

ResNet18

对应的就是[2,2,2,2]

def resnet18(pretrained=False, **kwargs):
    """Constructs a ResNet-18 model.
    Args:
        pretrained (bool): If True, returns a model pre-trained on ImageNet
    """
    model = ResNet(BasicBlock, [2, 2, 2, 2], **kwargs)
    if pretrained:
        print('Loading the pretrained model ...')
        # strict = False as we don't need fc layer params.
        model.load_state_dict(model_zoo.load_url(model_urls['resnet18']), strict=False)
    return model

ResNet34

def resnet34(pretrained=False, **kwargs):
    """Constructs a ResNet-34 model.
    Args:
        pretrained (bool): If True, returns a model pre-trained on ImageNet
    """
    model = ResNet(BasicBlock, [3, 4, 6, 3], **kwargs)
    if pretrained:
        print('Loading the pretrained model ...')
        model.load_state_dict(model_zoo.load_url(model_urls['resnet34']), strict=False)
    return model

ResNet20

这个需要强调一下,正常的ResNet20应该是文章中提出，针对cifar数据集设计的n=3时候, 1+6*3+1=20

class ResNet4Cifar(nn.Module):
    def __init__(self, block, num_block, num_classes=10):
        super().__init__()
        self.in_channels = 16
        self.conv1 = nn.Sequential(
            nn.Conv2d(3, 16, kernel_size=3, padding=1, bias=False),
            nn.BatchNorm2d(16),
            nn.ReLU(inplace=True))
        # we use a different inputsize than the original paper
        # so conv2_x's stride is 1
        self.conv2_x = self._make_layer(block, 16, num_block[0], 1)
        self.conv3_x = self._make_layer(block, 32, num_block[1], 2)
        self.conv4_x = self._make_layer(block, 64, num_block[2], 2)
        self.avg_pool = nn.AdaptiveAvgPool2d((1, 1))
        self.fc = nn.Linear(64 * block.expansion, num_classes)
    def _make_layer(self, block, out_channels, num_blocks, stride):
        strides = [stride] + [1] * (num_blocks - 1)
        layers = []
        for stride in strides:
            layers.append(block(self.in_channels, out_channels, stride))
            self.in_channels = out_channels * block.expansion
        return nn.Sequential(*layers)
    def forward(self, x):
        output = self.conv1(x)
        output = self.conv2_x(output)
        output = self.conv3_x(output)
        output = self.conv4_x(output)
        output = self.avg_pool(output)
        output = output.view(output.size(0), -1)
        output = self.fc(output)
        return output
def resnet20(num_classes=10, **kargs):
    """ return a ResNet 20 object
    """
    return ResNet4Cifar(BasicBlock, [3, 3, 3], num_classes=num_classes)

我们通过参数量的计算也为0.27M，和论文中的一致，对[1,3,32,32]的输入，输出维度为[1,64,8,8]

图2 ResNet20参数量计算

但是也有一些文章只换了开头三层的3x3卷积层，通道数并没有采用16、32、64，仍是4层的64、128、256、512
，这样下来参数量是11.25M。针对的任务不同，但是如果不关注原始网络结构，这一点可以忽略。

Bottleneck Block

Bottleneck Block中使用了1×1卷积层。如输入通道数为256，1×1卷积层会将通道数先降为64，经过3×3卷积层后，再将通道数升为256。1×1卷积层的优势是在更深的网络中，用较小的参数量处理通道数很大的输入。
这种结构用在ResNet50、ResNet101中。

图2. Bottleneck 结构图¹

class Bottleneck(nn.Module):
    expansion = 4
    def __init__(self, inplanes, planes, stride=1, downsample=None):
        super(Bottleneck, self).__init__()
        self.conv1 = conv1x1(inplanes, planes)
        self.bn1 = nn.BatchNorm2d(planes)
        self.conv2 = conv3x3(planes, planes, stride)
        self.bn2 = nn.BatchNorm2d(planes)
        self.conv3 = conv1x1(planes, planes * self.expansion)
        self.bn3 = nn.BatchNorm2d(planes * self.expansion)
        self.relu = nn.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

ResNet50

图3. ResNet50结构图²

和以上的网络结构一样，把Bottleneck按层数堆起来就可以了

def resnet50(pretrained=False, **kwargs):
    """Constructs a ResNet-50 model.
    Args:
        pretrained (bool): If True, returns a model pre-trained on ImageNet
    """
    model = ResNet(Bottleneck, [3, 4, 6, 3], **kwargs)
    if pretrained:
        print('Loading the pretrained model ...')
        model.load_state_dict(model_zoo.load_url(model_urls['resnet50']), strict=False)
    return model

ResNet到底解决了什么问题

推荐看知乎问题Resnet到底在解决一个什么问题呢？
贴一些我比较喜欢的回答：

A. 对于$L$层的网络来说，没有残差表示的Plain Net梯度相关性的衰减在
1
2
L
\frac{1}{2^L}
2L1​ ，而ResNet的衰减却只有
1
L
\frac{1}{\sqrt{L}}
L​1​ 。即使BN过后梯度的模稳定在了正常范围内，但梯度的相关性实际上是随着层数增加持续衰减的。而经过证明，ResNet可以有效减少这种相关性的衰减。

B. 对于“梯度弥散”观点来说，在输出引入一个输入x的恒等映射，则梯度也会对应地引入一个常数1，这样的网络的确不容易出现梯度值异常，在某种意义上，起到了稳定梯度的作用。

C. 跳连接相加可以实现不同分辨率特征的组合，因为浅层容易有高分辨率但是低级语义的特征，而深层的特征有高级语义，但分辨率就很低了。引入跳接实际上让模型自身有了更加“灵活”的结构，即在训练过程本身，模型可以选择在每一个部分是“更多进行卷积与非线性变换”还是“更多倾向于什么都不做”，抑或是将两者结合。模型在训练便可以自适应本身的结构。³

D. 当使用了残差网络时,就是加入了skip connection 结构,这时候由一个building block 的任务由: F(x) := H(x)，变成了F(x) := H(x)-x对比这两个待拟合的函数, 拟合残差图更容易优化,也就是说:F(x) := H(x)-x比F(x) := H(x)更容易优化⁴. 举了一个差分放大器的例子：F是求和前网络映射，H是从输入到求和后的网络映射。比如把5映射到5.1，那么引入残差前是F’(5)=5.1，引入残差后是H(5)=5.1, H(5)=F(5)+5, F(5)=0.1。这里的F’和F都表示网络参数映射，引入残差后的映射对输出的变化更敏感。比如s输出从5.1变到5.2，映射F’的输出增加了1/51=2%，而对于残差结构输出从5.1到5.2，映射F是从0.1到0.2，增加了100%。明显后者输出变化对权重的调整作用更大，所以效果更好。残差的思想都是去掉相同的主体部分，从而突出微小的变化。

说法众多，好用就完事儿了嗷~