ResNet

目的

Resnet网络是为了解决深度网络中的退化问题，即网络层数越深时，在数据集上表现的性能却越差。

原理

ResNet的单元结构如下：

类似动态规划的选择性继承，同时会在训练过程中逐渐增大（/缩小）该单元中权重层的参数，主要取决于是否是直接继承前面块更优。

实现

对于ResNet50及以上来说，采用的单元块是Bottleneck模块。

在实现Bottleneck模块前，需要先对ResNet中使用到的卷积核进行简化定义。

1. 首先是卷积核kernel_sizef分别为1和3的定义：

   def conv1x1(in_channel, out_channel, stride=1):
       return nn.Conv2d(in_channel, out_channel, kernel_size=1, stride=stride, bias=False)
   
   def conv3x3(in_channel, out_channel, stride=1):
       return nn.Conv2d(in_channel, out_channel, kernel_size=3, stride=stride, padding=1, bias=False)
   

2. 接着定义Bottleneck单元模块：

   这里有一个涉及到梯度是否能计算的问题，如果在bn3之后又进行了一次relu操作，然后再自加等，由于relu操作是限定为原地进行的，这就会导致在反向推导时无法计算出梯度，具体原因有待考究。

   class Bottleneck(nn.Module):
       extension = 4
   
       # Bottleneck only decrease the [h,w] in conv1 when stride > 1,
       # so the [h,w] is to be [(h-1)/stride+1,(w-1)/stride+1].
       # the in_channel will be change to channel*extension.
       # channel is the temp variable.
       def __init__(self, in_channel, channel, stride, downsample=None):
           super(Bottleneck, self).__init__()
   
           self.conv1 = conv1x1(in_channel, channel, stride)
           self.bn1 = nn.BatchNorm2d(channel)
   
           self.conv2 = conv3x3(channel, channel)
           self.bn2 = nn.BatchNorm2d(channel)
   
           self.conv3 = conv1x1(channel, channel * self.extension)
           self.bn3 = nn.BatchNorm2d(channel * self.extension)
   
           self.relu = nn.ReLU(inplace=True)
           self.downsample = downsample
           self.stride = stride
   
       def forward(self, x):
           identity = x
           x = self.conv1(x)
           x = self.bn1(x)
           x = self.relu(x)
   
           x = self.conv2(x)
           x = self.bn2(x)
           x = self.relu(x)
   
           x = self.conv3(x)
           x = self.bn3(x)
   
           if self.downsample is not None:
               identity = self.downsample(identity)
   
           x += identity
           x = self.relu(x)
           return x
   

3. 最后是ResNet的主体，主体包含前向传播函数和构造集合体模块层函数：

ResNet总共可看作6层结构。

• 第一层为大卷积核层，主要是以大卷积核进行卷积，同时将通道数上升到64。[h,w]=[h/2,w/2]。

• 第二至五层是残差模块，其中残差模块由多层Bottleneck组成。多层Bottleneck的第一层的in_channel为上一个模块的out_channel，中间的in_channel则为多层Bottleneck的上一层out_channel，每个Bottleneck的plane为其in_channel的1/2。

  第二层的stride为1，但是有maxpool来使得图片尺寸缩小，其他层则通过stride=2使得图片尺寸缩小。

• 第六层则是全连接层，使用torch.flatten进行缩维度处理。

class ResNet(nn.Module):
    # size / 32 / 7
    def __init__(self, block, layers, num_class):
        super(ResNet, self).__init__()
        # the first layer changes the channel to 64,
        # and the [h,w] will be change to [(h-1)/stride+1,(w-1)/stride+1] after the first layer.
        self.in_channel = 64
        self.block = block
        self.layers = layers

        self.conv1 = nn.Conv2d(3, self.in_channel, kernel_size=7, stride=2, padding=3, bias=False)
        self.bn1 = nn.BatchNorm2d(self.in_channel)
        self.relu = nn.ReLU()
        self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)

        # there are four block layers, each layers contains more than one block.
        self.stage1 = self.make_layer(self.block, 64, layers[0], stride=1)
        self.stage2 = self.make_layer(self.block, 128, layers[1], stride=2)
        self.stage3 = self.make_layer(self.block, 256, layers[2], stride=2)
        self.stage4 = self.make_layer(self.block, 512, layers[3], stride=2)

        # in the end, there will be a linear layer to classify all the classes.
        # self.avgpool = nn.AvgPool2d(7)
        self.fc = nn.Linear(512 * block.extension, num_class)

    def make_layer(self, block, plane, block_num, stride=1):
        block_list = []
        downsample = None

        # if the in_channel isn't equal to the out_channel,
        # downsample will be needed to process the in_channel to same size as the out_channel
        # so that the in_channel can be added to the out_channel to achieve the resnet struct.
        if stride != 1 or self.in_channel != plane * block.extension:
            downsample = nn.Sequential(
                conv1x1(self.in_channel, plane * block.extension, stride),
                nn.BatchNorm2d(plane * block.extension)
            )

        conv_block = block(self.in_channel, plane, stride, downsample=downsample)

        # the first block's in_channel is different to the another block_num-1 in_channel.
        block_list.append(conv_block)
        # modify the in_channel for the next stage layer.
        self.in_channel = plane * block.extension

        for _ in range(1, block_num):
            block_list.append(block(self.in_channel, plane, stride=1))

        return nn.Sequential(*block_list)

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

        x = self.stage1(x)
        x = self.stage2(x)
        x = self.stage3(x)
        x = self.stage4(x)

        # x = self.avgpool(x)
        x = torch.flatten(x, 1)
        x = self.fc(x)
        x = nn.Softmax(dim=1)(x)
        return x