CIFAR-10 数据集实战——构建ResNet18神经网络
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如果不了解ResNet的同学可以先看我的这篇博客ResNet论文阅读
首先实现一个Residual Block
import torch from torch import nn from torch.nn import functional as F class ResBlk(nn.Module): def __init__(self, ch_in, ch_out, stride=1): super(ResBlk, self).__init__() self.conv1 = nn.Conv2d(ch_in, ch_out, kernel_size=3, stride=stride, padding=1) self.bn1 = nn.BatchNorm2d(ch_out) self.conv2 = nn.Conv2d(ch_out, ch_out, kernel_size=3, stride=1, padding=1) self.bn2 = nn.BatchNorm2d(ch_out) if ch_out == ch_in: self.extra = nn.Sequential() else: self.extra = nn.Sequential( # 1×1的卷积作用是修改输入x的channel # [b, ch_in, h, w] => [b, ch_out, h, w] nn.Conv2d(ch_in, ch_out, kernel_size=1, stride=stride), nn.BatchNorm2d(ch_out), ) def forward(self, x): out = F.relu(self.bn1(self.conv1(x))) out = self.bn2(self.conv2(out)) # short cut out = self.extra(x) + out out = F.relu(out) return outBlock中进行了正则化处理，以使train过程更快更稳定。同时要考虑，如果两元素的ch_in和ch_out不匹配，进行加法时会报错，因此需要判断一下，如果不想等，就用1×1的卷积调整一下
测试一下
blk = ResBlk(64, 128, stride=2) tmp = torch.randn(2, 64, 32, 32) out = blk(tmp) print(out.shape)输出的shape大小是
torch.Size([2, 128, 16, 16])这里解释一下，为什么有的层要专门设置stride。先不考虑别的层，对于一个Residual block，channel从64增大到128，如果所有的stride都是1，padding也是1，那么图片的w和h也不会变，但是channel增大了，此时就会导致整个网络的参数增多。而这才仅仅一个Block，更不用说后面的FC以及更多Block了，所以stride不能全部设置为1，不要让网络的参数一直增大
然后我们搭建完整的ResNet-18
class ResNet18(nn.Module): def __init__(self): super(ResNet18, self).__init__() self.conv1 = nn.Sequential( nn.Conv2d(3, 64, kernel_size=3, stride=3, padding=0), nn.BatchNorm2d(64), ) # followed 4 blocks # [b, 64, h, w] => [b, 128, h, w] self.blk1 = ResBlk(64, 128, stride=2) # [b, 128, h, w] => [b, 256, h, w] self.blk2 = ResBlk(128, 256, stride=2) # [b, 256, h, w] => [b, 512, h, w] self.blk3 = ResBlk(256, 512, stride=2) # [b, 512, h, w] => [b, 512, h, w] self.blk4 = ResBlk(512, 512, stride=2) self.outlayer = nn.Linear(512*1*1, 10) def forward(self, x): x = F.relu(self.conv1(x)) # 经过四个blk以后 [b, 64, h, w] => [b, 512, h, w] x = self.blk1(x) x = self.blk2(x) x = self.blk3(x) x = self.blk4(x) x = self.outlayer(x) return x测试一下
x = torch.randn(2, 3, 32, 32) model = ResNet18() out = model(x) print("ResNet:", out.shape)结果报错了，错误信息如下
size mismatch, m1: [2048 x 2], m2: [512 x 10] at /pytorch/aten/src/TH/generic/THTensorMath.cpp:961问题在于我们最后定义线性层的输入维度，和上一层Block的输出维度不匹配，在ResNet18的最后一个Block运行结束后打印一下当前x的shape，结果是
torch.Size([2, 512, 2, 2])解决办法有很多，可以修改线性层的输入进行匹配，也可以在最后一层Block后面再进行一些操作，使其与512匹配
先给出修改后的代码，在做解释
class ResNet18(nn.Module): def __init__(self): super(ResNet18, self).__init__() self.conv1 = nn.Sequential( nn.Conv2d(3, 64, kernel_size=3, stride=3, padding=0), nn.BatchNorm2d(64), ) # followed 4 blocks # [b, 64, h, w] => [b, 128, h, w] self.blk1 = ResBlk(64, 128, stride=2) # [b, 128, h, w] => [b, 256, h, w] self.blk2 = ResBlk(128, 256, stride=2) # [b, 256, h, w] => [b, 512, h, w] self.blk3 = ResBlk(256, 512, stride=2) # [b, 512, h, w] => [b, 512, h, w] self.blk4 = ResBlk(512, 512, stride=2) self.outlayer = nn.Linear(512*1*1, 10) def forward(self, x): x = F.relu(self.conv1(x)) # 经过四个blk以后 [b, 64, h, w] => [b, 512, h, w] x = self.blk1(x) x = self.blk2(x) x = self.blk3(x) x = self.blk4(x) # print("after conv:", x.shape) # [b, 512, 2, 2] # [b, 512, h, w] => [b, 512, 1, 1] x = F.adaptive_avg_pool2d(x, [1, 1]) x = x.view(x.size(0), -1) # [b, 512, 1, 1] => [b, 512*1*1] x = self.outlayer(x) return x这里我采用的是第二种方法，在最后一个Block结束以后，接了一个自适应的pooling层，这个pooling的作用是将不论输入的宽高是多少，全部输出称宽高都是1的tensor，其他维度保持不变。然后再做一个reshape操作，将
[batchsize, 512, 1, 1]reshape成[batchsize, 512*1*1]大小的tensor，这样就和接下来的线性层对上了，线性层的输入大小是512，输出是10。因此整个网络最终输出的shape就是[batchsize, 10]最后我们把之前训练LeNet5的代码拷贝过来，将里面的
model=LeNet5()改为model=ResNet18()就行了。完整代码如下import torch from torch import nn, optim import torch.nn.functional as F from torch.utils.data import DataLoader from torchvision import datasets, transforms batch_size=32 cifar_train = datasets.CIFAR10(root='cifar', train=True, transform=transforms.Compose([ transforms.Resize([32, 32]), transforms.ToTensor(), ]), download=True) cifar_train = DataLoader(cifar_train, batch_size=batch_size, shuffle=True) cifar_test = datasets.CIFAR10(root='cifar', train=False, transform=transforms.Compose([ transforms.Resize([32, 32]), transforms.ToTensor(), ]), download=True) cifar_test = DataLoader(cifar_test, batch_size=batch_size, shuffle=True) class ResBlk(nn.Module): def __init__(self, ch_in, ch_out, stride=1): super(ResBlk, self).__init__() self.conv1 = nn.Conv2d(ch_in, ch_out, kernel_size=3, stride=stride, padding=1) self.bn1 = nn.BatchNorm2d(ch_out) self.conv2 = nn.Conv2d(ch_out, ch_out, kernel_size=3, stride=1, padding=1) self.bn2 = nn.BatchNorm2d(ch_out) if ch_out == ch_in: self.extra = nn.Sequential() else: self.extra = nn.Sequential( # 1×1的卷积作用是修改输入x的channel # [b, ch_in, h, w] => [b, ch_out, h, w] nn.Conv2d(ch_in, ch_out, kernel_size=1, stride=stride), nn.BatchNorm2d(ch_out), ) def forward(self, x): out = F.relu(self.bn1(self.conv1(x))) out = self.bn2(self.conv2(out)) # short cut out = self.extra(x) + out out = F.relu(out) return out class ResNet18(nn.Module): def __init__(self): super(ResNet18, self).__init__() self.conv1 = nn.Sequential( nn.Conv2d(3, 64, kernel_size=3, stride=3, padding=0), nn.BatchNorm2d(64), ) # followed 4 blocks # [b, 64, h, w] => [b, 128, h, w] self.blk1 = ResBlk(64, 128, stride=2) # [b, 128, h, w] => [b, 256, h, w] self.blk2 = ResBlk(128, 256, stride=2) # [b, 256, h, w] => [b, 512, h, w] self.blk3 = ResBlk(256, 512, stride=2) # [b, 512, h, w] => [b, 512, h, w] self.blk4 = ResBlk(512, 512, stride=2) self.outlayer = nn.Linear(512*1*1, 10) def forward(self, x): x = F.relu(self.conv1(x)) # 经过四个blk以后 [b, 64, h, w] => [b, 512, h, w] x = self.blk1(x) x = self.blk2(x) x = self.blk3(x) x = self.blk4(x) # print("after conv:", x.shape) # [b, 512, 2, 2] # [b, 512, h, w] => [b, 512, 1, 1] x = F.adaptive_avg_pool2d(x, [1, 1]) x = x.view(x.size(0), -1) # [b, 512, 1, 1] => [b, 512*1*1] x = self.outlayer(x) return x def main(): ########## train ########## #device = torch.device('cuda') #model = ResNet18().to(device) criteon = nn.CrossEntropyLoss() model = ResNet18() optimizer = optim.Adam(model.parameters(), 1e-3) for epoch in range(1000): model.train() for batchidx, (x, label) in enumerate(cifar_train): #x, label = x.to(device), label.to(device) logits = model(x) # logits: [b, 10] # label: [b] loss = criteon(logits, label) # backward optimizer.zero_grad() loss.backward() optimizer.step() print('train:', epoch, loss.item()) ########## test ########## model.eval() with torch.no_grad(): total_correct = 0 total_num = 0 for x, label in cifar_test: # x, label = x.to(device), label.to(device) # [b] logits = model(x) # [b] pred = logits.argmax(dim=1) # [b] vs [b] total_correct += torch.eq(pred, label).float().sum().item() total_num += x.size(0) acc = total_correct / total_num print('test:', epoch, acc) if __name__ == '__main__': main()
ResNet和LeNet相比，准确率提升的很快，但是由于层数增加，不可避免的会导致运行时间增加，如果没有GPU，运行一个epoch大概要15分钟。读者同样可以在此基础上修改网络结构，运用一些tricks，比方说一开始就对图片做一个Normalize等
