目录

• 主要内容
• 代码

He K, Zhang X, Ren S, et al. Deep Residual Learning for Image Recognition[C]. computer vision and pattern recognition, 2016: 770-778.

@article{he2016deep,

title={Deep Residual Learning for Image Recognition},

author={He, Kaiming and Zhang, Xiangyu and Ren, Shaoqing and Sun, Jian},

pages={770--778},

year={2016}}

深度一直是CNN很重要的一个点, 作者发现, 当仅仅增加层数不一定会带来优势, 甚至会误差会加大, 而且这个误差并非是过拟合导致的.

设输入为\(x\), 一般的网络的输出可以表示为\(\mathcal{H}(x)\), 作者考虑的是

\[\tag{1}
\mathcal{F}(x):=\mathcal{H}(x)-x.
\]

实际上看到这里是有困惑的, 为什么\(\mathcal{H}(x)-x\)是成立的? 这不就意味着网络的输出和输入是同样大小的? 那还怎么分类.

从上面的图中可以看到, 其实\(\mathcal{H}(x)\)并非是整个网络的输出, 而是某些层的输出，图中每俩个层就会进行一次残差的操作. 所以用网络去学习\(\mathcal{F}(x)\), 能够把前者的信息更好的传递下去. 就像作者说的, 如果前面部分的层能够很好的完成任务, 后面的层只需要称为恒等映射就行了. 但是恒等映射不一定能够被很好的逼近, 这将导致网络加深反而误差变大, 但是如果改成学习残差就很容易了, 因为后面的层只需要将权重设置为0，那么后面每一块的输出都会是\(x\)(为某一层的输出), 这至少能够保证深度加深结果不会变坏.

当然还有最后一个问题, \(x\)的大小终究是要变化的, 所以我们没法保证\(\mathcal{F}(x)\)和\(x\)的尺寸是一致的, 一种解决办法是增加一个线性映射

\[\tag{2}
\mathcal{F}(x)+W_s x,
\]

代码里用的便是1x1的卷积核, 或者也可以通过补零来实现.

"""
Resnet34训练于CIFAR10
epoches=1000
lr=0.01 论文中0.1开始  试了以下梯度炸了 可能是网络结构的原因
momentum=0.9
weight_decay=0.0001
"""

import torch
import torch.nn as nn
import torchvision
import torchvision.transforms as transforms
import numpy as np
import os

class Residualblock(nn.Module):

    def __init__(self, in_channels, out_channels,
                 stride=1, shortcut=None):
        super(Residualblock, self).__init__()

        self.longway = nn.Sequential(
            nn.Conv2d(in_channels, out_channels, 3, stride, 1),
            nn.BatchNorm2d(out_channels),
            nn.ReLU(inplace=True),
            nn.Conv2d(out_channels, out_channels, 3, 1, 1),
            nn.BatchNorm2d(out_channels),
            nn.ReLU(inplace=True)
        )

        self.shortway = shortcut

    def forward(self, x):

        residual = self.longway(x)
        identity = x if self.shortway is None else self.shortway(x)
        return nn.functional.relu(identity + residual)

class ResNet(nn.Module):

    def __init__(self, out_size=10, layers=None):
        """
        :param out_size: 输出的类的数量
        :param layers:  每组有多少块 说不清 回看论文
        """
        super(ResNet, self).__init__()

        if layers is None:
            layers = (2, 3, 5, 2)
        self.conv1 = nn.Sequential(
            nn.Conv2d(3, 64, 7, 2, 3),
            nn.BatchNorm2d(64),
            nn.ReLU(inplace=True),
            nn.MaxPool2d(3, 2, 1)
        )
        self.layer1 = self._make_layer(64, 64, layers[0])
        self.layer2 = self._make_layer(64, 128, layers[1], 2)
        self.layer3 = self._make_layer(128, 256, layers[2], 2)
        self.layer4 = self._make_layer(256, 512, layers[3], 2)

        #ada_avg: 将输入(N, C, H, W) -> (N, C, H*, W*)
        #下面H*, W* = 1, 1
        self.avg_pool = nn.AdaptiveAvgPool2d((1, 1))
        self.fc = nn.Linear(512, out_size)

        #直接从pytorch源码中搬来的初始化
        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.GroupNorm)):
                nn.init.constant_(m.weight, 1)
                nn.init.constant_(m.bias, 0)

    def _make_layer(self, in_channels, out_channels,
                    block_nums, stride=1):

        shortcut = nn.Sequential(
            nn.Conv2d(in_channels, out_channels, 1, stride)
        )
        layer = [nn.Sequential(
            Residualblock(in_channels, out_channels, stride, shortcut)
        )]
        for block in range(block_nums):
            layer.append(
                Residualblock(out_channels, out_channels, 1)
            )
        return nn.Sequential(*layer)

    def forward(self, x):

        x = self.conv1(x)

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

        x = torch.flatten(x, 1) #展平 等价于.vier(x.size(0), -1)
        out = self.fc(x)
        return out

class Train:

    def __init__(self, lr=0.01, momentum=0.9, weight_decay=0.0001):
        self.net = ResNet()
        self.criterion = nn.CrossEntropyLoss()
        self.opti = torch.optim.SGD(self.net.parameters(),
                                    lr=lr, momentum=momentum,
                                    weight_decay=weight_decay)
        self.gpu()
        self.generate_path()
        self.acc_rates = []
        self.errors = []

    def gpu(self):
        self.device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
        if torch.cuda.device_count() > 1:
            print("Let'us use %d GPUs" % torch.cuda.device_count())
            self.net = nn.DataParallel(self.net)
        self.net = self.net.to(self.device)

    def generate_path(self):
        """
        生成保存数据的路径
        :return:
        """
        try:
            os.makedirs('./paras')
            os.makedirs('./logs')
            os.makedirs('./infos')
        except FileExistsError as e:
            pass
        name = self.net.__class__.__name__
        paras = os.listdir('./paras')
        logs = os.listdir('./logs')
        infos = os.listdir('./infos')
        number = max((len(paras), len(logs), len(infos)))
        self.para_path = "./paras/{0}{1}.pt".format(
            name,
            number
        )

        self.log_path = "./logs/{0}{1}.txt".format(
            name,
            number
        )
        self.info_path = "./infos/{0}{1}.npy".format(
            name,
            number
        )

    def log(self, strings):
        """
        运行日志
        :param strings:
        :return:
        """
        # a 往后添加内容
        with open(self.log_path, 'a', encoding='utf8') as f:
            f.write(strings)

    def save(self):
        """
        保存网络参数
        :return:
        """
        torch.save(self.net.state_dict(), self.para_path)

    def derease_lr(self, multi=10):
        """
        降低学习率
        :param multi:
        :return:
        """
        self.opti.param_groups()[0]['lr'] /= multi

    def train(self, trainloder, epochs=50):
        data_size = len(trainloder) * trainloder.batch_size
        part = int(trainloder.batch_size / 2)
        for epoch in range(epochs):
            running_loss = 0.
            total_loss = 0.
            acc_count = 0.
            if (epoch + 1) % int(epochs / 2) is 0:
                self.derease_lr()
                self.log(#日志记录
                    "learning rate change!!!\n"
                )
            for i, data in enumerate(trainloder):
                imgs, labels = data
                imgs = imgs.to(self.device)
                labels = labels.to(self.device)
                out = self.net(imgs)
                loss = self.criterion(out, labels)
                _, pre = torch.max(out, 1)  #判断是否判断正确
                acc_count += (pre == labels).sum().item() #加总对的个数

                self.opti.zero_grad()
                loss.backward()
                self.opti.step()

                running_loss += loss.item()

                if (i+1) % part is 0:
                    strings = "epoch {0:<3} part {1:<5} loss: {2:<.7f}\n".format(
                        epoch, i, running_loss / part
                    )
                    self.log(strings)#日志记录
                    total_loss += running_loss
                    running_loss = 0.
            self.acc_rates.append(acc_count / data_size)
            self.errors.append(total_loss / data_size)
            self.log( #日志记录
                "Accuracy of the network on %d train images: %d %%\n" %(
                    data_size, acc_count / data_size * 100
                )
            )
            self.save() #保存网络参数
        #保存一些信息画图用
        np.save(self.info_path, {
            'acc_rates': np.array(self.acc_rates),
            'errors': np.array(self.errors)
        })

if __name__ == "__main__":

    root = "../../data"

    trainset = torchvision.datasets.CIFAR10(root=root, train=True,
                                          download=False,
                                          transform=transforms.Compose(
                                              [transforms.ToTensor(),
                                               transforms.Normalize((0.5, 0.5, 0.5), (0.5, 0.5, 0.5))]
                                          ))

    train_loader = torch.utils.data.DataLoader(trainset, batch_size=128,
                                              shuffle=True, num_workers=0)

    dog = Train()
    dog.train(train_loader, epochs=1000)