《动手学深度学习》Paddle 版源码-5.11章（ResNet）-人工智能-PHP中文网

残差网络（ResNet）由何恺明等人提出，解决了深层神经网络训练误差不降反升的问题。其核心是残差块，通过拟合残差映射简化优化，输入可跨层传播。ResNet沿用VGG的3×3卷积设计，含4个残差块模块，结构简单。如ResNet-18有18层，还有更深的型号。在Fashion-MNIST上训练验证了其有效性，深刻影响了深度神经网络设计。

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《动手学深度学习》paddle 版源码-5.11章（resnet） - php中文网

残差网络（ResNet）

让我们先思考一个问题：对神经网络模型添加新的层，充分训练后的模型是否只可能更有效地降低训练误差？理论上，原模型解的空间只是新模型解的空间的子空间。也就是说，如果我们能将新添加的层训练成恒等映射 $f (x) = x$ f(x)=x，新模型和原模型将同样有效。由于新模型可能得出更优的解来拟合训练数据集，因此添加层似乎更容易降低训练误差。然而在实践中，添加过多的层后训练误差往往不降反升。即使利用批量归一化带来的数值稳定性使训练深层模型更加容易，该问题仍然存在。针对这一问题，何恺明等人提出了残差网络（ResNet） [1]。它在2015年的ImageNet图像识别挑战赛夺魁，并深刻影响了后来的深度神经网络的设计。

残差块

让我们聚焦于神经网络局部。如图5.9所示，设输入为 $x$ x。假设我们希望学出的理想映射为 $f (x)$ f(x)，从而作为图5.9上方激活函数的输入。左图虚线框中的部分需要直接拟合出该映射 $f (x)$ f(x)，而右图虚线框中的部分则需要拟合出有关恒等映射的残差映射 $f (x) - x$ f(x)−x。残差映射在实际中往往更容易优化。以本节开头提到的恒等映射作为我们希望学出的理想映射 $f (x)$ f(x)。我们只需将图5.9中右图虚线框内上方的加权运算（如仿射）的权重和偏差参数学成0，那么 $f (x)$ f(x)即为恒等映射。实际中，当理想映射 $f (x)$ f(x)极接近于恒等映射时，残差映射也易于捕捉恒等映射的细微波动。图5.9右图也是ResNet的基础块，即残差块（residual block）。在残差块中，输入可通过跨层的数据线路更快地向前传播。

《动手学深度学习》Paddle 版源码-5.11章（ResNet） - php中文网

ResNet沿用了VGG全 $3 \times 3$ 3×3卷积层的设计。残差块里首先有2个有相同输出通道数的 $3 \times 3$ 3×3卷积层。每个卷积层后接一个批量归一化层和ReLU激活函数。然后我们将输入跳过这2个卷积运算后直接加在最后的ReLU激活函数前。这样的设计要求2个卷积层的输出与输入形状一样，从而可以相加。如果想改变通道数，就需要引入一个额外的 $1 \times 1$ 1×1卷积层来将输入变换成需要的形状后再做相加运算。

残差块的实现如下。它可以设定输出通道数、是否使用额外的 $1 \times 1$ 1×1卷积层来修改通道数以及卷积层的步幅。

In [ ]

import paddleimport paddle.nn as nnimport numpy as npimport warnings
warnings.filterwarnings("ignore", category=Warning) # 过滤报警信息class Residual(nn.Layer):
    def __init__(self, num_channels, num_filters, use_1x1conv=False, stride=1):
        super(Residual, self).__init__()
        self.use_1x1conv = use_1x1conv
        model = [
            nn.Conv2D(num_channels, num_filters, 3, stride=stride, padding=1),
            nn.BatchNorm2D(num_filters),
            nn.ReLU(),
            nn.Conv2D(num_filters, num_filters, 3, stride=1, padding=1),
            nn.BatchNorm2D(num_filters),
        ]
        self.model = nn.Sequential(*model)        if use_1x1conv:
            model_1x1 = [nn.Conv2D(num_channels, num_filters, 1, stride=stride)]
            self.model_1x1 = nn.Sequential(*model_1x1)    def forward(self, X):
        Y = self.model(X)        if self.use_1x1conv:
            X = self.model_1x1(X)        return paddle.nn.functional.relu(X + Y)

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下面我们来查看输入和输出形状一致的情况。

In [ ]

blk = Residual(3, 3)
X = paddle.to_tensor(np.random.uniform(-1., 1., [4, 3, 6, 6]).astype('float32'))
Y = blk(X)print(Y.shape)

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[4, 3, 6, 6]

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我们也可以在增加输出通道数的同时减半输出的高和宽。

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In [ ]

blk = Residual(3, 6, use_1x1conv=True, stride=2)
X = paddle.to_tensor(np.random.uniform(-1., 1., [4, 3, 6, 6]).astype('float32'))
Y = blk(X)print(Y.shape)

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[4, 6, 3, 3]

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GoogLeNet在后面接了4个由Inception块组成的模块。ResNet则使用4个由残差块组成的模块，每个模块使用若干个同样输出通道数的残差块。第一个模块的通道数同输入通道数一致。由于之前已经使用了步幅为2的最大池化层，所以无须减小高和宽。之后的每个模块在第一个残差块里将上一个模块的通道数翻倍，并将高和宽减半。

下面我们来实现这个模块。注意，这里对第一个模块做了特别处理。

In [ ]

class ResnetBlock(nn.Layer):
    def __init__(self, num_channels, num_filters, num_residuals, first_block=False):
        super(ResnetBlock, self).__init__()
        model = []        for i in range(num_residuals):            if i == 0:                if not first_block:
                    model += [Residual(num_channels, num_filters, use_1x1conv=True, stride=2)]                else:
                    model += [Residual(num_channels, num_filters)]            else:
                model += [Residual(num_filters, num_filters)]
        self.model = nn.Sequential(*model)    def forward(self, X):
        return self.model(X)

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ResNet模型

In [ ]

class ResNet(nn.Layer):
    def __init__(self, num_classes=10):
        super(ResNet, self).__init__()        # ResNet的前两层跟之前介绍的GoogLeNet中的一样：
        # 在输出通道数为64、步幅为2的7×77卷积层后接步幅为2的3×3的最大池化层。
        # 不同之处在于ResNet每个卷积层后增加的批量归一化层。
        model = [
            nn.Conv2D(1, 64, 7, stride=2, padding=3),
            nn.BatchNorm2D(64),
            nn.ReLU(),
            nn.MaxPool2D(kernel_size=3, stride=2, padding=1)
        ]        # 接着我们为ResNet加入所有残差块。这里每个模块使用2个残差块。
        model += [
            ResnetBlock(64, 64, 2, first_block=True),
            ResnetBlock(64, 128, 2),
            ResnetBlock(128, 256, 2),
            ResnetBlock(256, 512, 2)
        ]        # 最后，与GoogLeNet一样，加入全局平均池化层后接上全连接层输出。
        model += [
            nn.AdaptiveAvgPool2D(output_size=1),
            nn.Flatten(start_axis=1, stop_axis=-1),
            nn.Linear(512, num_classes),
        ]
        self.model = nn.Sequential(*model)    def forward(self, X):
        Y = self.model(X)        return Y

rn = ResNet()
X = paddle.to_tensor(np.random.uniform(-1., 1., [4, 1, 96, 96]).astype('float32'))
Y = rn(X)print(Y.shape)

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[4, 10]

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这里每个模块里有4个卷积层（不计算 $1 \times 1$ 1×1卷积层），加上最开始的卷积层和最后的全连接层，共计18层。这个模型通常也被称为ResNet-18。通过配置不同的通道数和模块里的残差块数可以得到不同的ResNet模型，例如更深的含152层的ResNet-152。虽然ResNet的主体架构跟GoogLeNet的类似，但ResNet结构更简单，修改也更方便。这些因素都导致了ResNet迅速被广泛使用。

在训练ResNet之前，我们来观察一下输入形状在ResNet不同模块之间的变化。

In [ ]

resnet = ResNet(10)
param_info = paddle.summary(resnet, (1, 1, 96, 96))print(param_info)

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-------------------------------------------------------------------------------
   Layer (type)         Input Shape          Output Shape         Param #    
===============================================================================
     Conv2D-26        [[1, 1, 96, 96]]     [1, 64, 48, 48]         3,200     
  BatchNorm2D-22     [[1, 64, 48, 48]]     [1, 64, 48, 48]          256      
      ReLU-12        [[1, 64, 48, 48]]     [1, 64, 48, 48]           0       
    MaxPool2D-2      [[1, 64, 48, 48]]     [1, 64, 24, 24]           0       
     Conv2D-27       [[1, 64, 24, 24]]     [1, 64, 24, 24]        36,928     
  BatchNorm2D-23     [[1, 64, 24, 24]]     [1, 64, 24, 24]          256      
      ReLU-13        [[1, 64, 24, 24]]     [1, 64, 24, 24]           0       
     Conv2D-28       [[1, 64, 24, 24]]     [1, 64, 24, 24]        36,928     
  BatchNorm2D-24     [[1, 64, 24, 24]]     [1, 64, 24, 24]          256      
    Residual-11      [[1, 64, 24, 24]]     [1, 64, 24, 24]           0       
     Conv2D-29       [[1, 64, 24, 24]]     [1, 64, 24, 24]        36,928     
  BatchNorm2D-25     [[1, 64, 24, 24]]     [1, 64, 24, 24]          256      
      ReLU-14        [[1, 64, 24, 24]]     [1, 64, 24, 24]           0       
     Conv2D-30       [[1, 64, 24, 24]]     [1, 64, 24, 24]        36,928     
  BatchNorm2D-26     [[1, 64, 24, 24]]     [1, 64, 24, 24]          256      
    Residual-12      [[1, 64, 24, 24]]     [1, 64, 24, 24]           0       
   ResnetBlock-5     [[1, 64, 24, 24]]     [1, 64, 24, 24]           0       
     Conv2D-31       [[1, 64, 24, 24]]     [1, 128, 12, 12]       73,856     
  BatchNorm2D-27     [[1, 128, 12, 12]]    [1, 128, 12, 12]         512      
      ReLU-15        [[1, 128, 12, 12]]    [1, 128, 12, 12]          0       
     Conv2D-32       [[1, 128, 12, 12]]    [1, 128, 12, 12]       147,584    
  BatchNorm2D-28     [[1, 128, 12, 12]]    [1, 128, 12, 12]         512      
     Conv2D-33       [[1, 64, 24, 24]]     [1, 128, 12, 12]        8,320     
    Residual-13      [[1, 64, 24, 24]]     [1, 128, 12, 12]          0       
     Conv2D-34       [[1, 128, 12, 12]]    [1, 128, 12, 12]       147,584    
  BatchNorm2D-29     [[1, 128, 12, 12]]    [1, 128, 12, 12]         512      
      ReLU-16        [[1, 128, 12, 12]]    [1, 128, 12, 12]          0       
     Conv2D-35       [[1, 128, 12, 12]]    [1, 128, 12, 12]       147,584    
  BatchNorm2D-30     [[1, 128, 12, 12]]    [1, 128, 12, 12]         512      
    Residual-14      [[1, 128, 12, 12]]    [1, 128, 12, 12]          0       
   ResnetBlock-6     [[1, 64, 24, 24]]     [1, 128, 12, 12]          0       
     Conv2D-36       [[1, 128, 12, 12]]     [1, 256, 6, 6]        295,168    
  BatchNorm2D-31      [[1, 256, 6, 6]]      [1, 256, 6, 6]         1,024     
      ReLU-17         [[1, 256, 6, 6]]      [1, 256, 6, 6]           0       
     Conv2D-37        [[1, 256, 6, 6]]      [1, 256, 6, 6]        590,080    
  BatchNorm2D-32      [[1, 256, 6, 6]]      [1, 256, 6, 6]         1,024     
     Conv2D-38       [[1, 128, 12, 12]]     [1, 256, 6, 6]        33,024     
    Residual-15      [[1, 128, 12, 12]]     [1, 256, 6, 6]           0       
     Conv2D-39        [[1, 256, 6, 6]]      [1, 256, 6, 6]        590,080    
  BatchNorm2D-33      [[1, 256, 6, 6]]      [1, 256, 6, 6]         1,024     
      ReLU-18         [[1, 256, 6, 6]]      [1, 256, 6, 6]           0       
     Conv2D-40        [[1, 256, 6, 6]]      [1, 256, 6, 6]        590,080    
  BatchNorm2D-34      [[1, 256, 6, 6]]      [1, 256, 6, 6]         1,024     
    Residual-16       [[1, 256, 6, 6]]      [1, 256, 6, 6]           0       
   ResnetBlock-7     [[1, 128, 12, 12]]     [1, 256, 6, 6]           0       
     Conv2D-41        [[1, 256, 6, 6]]      [1, 512, 3, 3]       1,180,160   
  BatchNorm2D-35      [[1, 512, 3, 3]]      [1, 512, 3, 3]         2,048     
      ReLU-19         [[1, 512, 3, 3]]      [1, 512, 3, 3]           0       
     Conv2D-42        [[1, 512, 3, 3]]      [1, 512, 3, 3]       2,359,808   
  BatchNorm2D-36      [[1, 512, 3, 3]]      [1, 512, 3, 3]         2,048     
     Conv2D-43        [[1, 256, 6, 6]]      [1, 512, 3, 3]        131,584    
    Residual-17       [[1, 256, 6, 6]]      [1, 512, 3, 3]           0       
     Conv2D-44        [[1, 512, 3, 3]]      [1, 512, 3, 3]       2,359,808   
  BatchNorm2D-37      [[1, 512, 3, 3]]      [1, 512, 3, 3]         2,048     
      ReLU-20         [[1, 512, 3, 3]]      [1, 512, 3, 3]           0       
     Conv2D-45        [[1, 512, 3, 3]]      [1, 512, 3, 3]       2,359,808   
  BatchNorm2D-38      [[1, 512, 3, 3]]      [1, 512, 3, 3]         2,048     
    Residual-18       [[1, 512, 3, 3]]      [1, 512, 3, 3]           0       
   ResnetBlock-8      [[1, 256, 6, 6]]      [1, 512, 3, 3]           0       
AdaptiveAvgPool2D-2   [[1, 512, 3, 3]]      [1, 512, 1, 1]           0       
     Flatten-3        [[1, 512, 1, 1]]         [1, 512]              0       
     Linear-2            [[1, 512]]            [1, 10]             5,130     
===============================================================================
Total params: 11,186,186
Trainable params: 11,170,570
Non-trainable params: 15,616
-------------------------------------------------------------------------------
Input size (MB): 0.04
Forward/backward pass size (MB): 10.77
Params size (MB): 42.67
Estimated Total Size (MB): 53.47
-------------------------------------------------------------------------------

{'total_params': 11186186, 'trainable_params': 11170570}

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训练模型

下面我们在Fashion-MNIST数据集上训练ResNet。

In [ ]

import paddleimport paddle.vision.transforms as Tfrom paddle.vision.datasets import FashionMNIST# 数据集处理transform = T.Compose([
    T.Resize(96),
    T.Transpose(),
    T.Normalize([127.5], [127.5]),
])
train_dataset = FashionMNIST(mode='train', transform=transform)
val_dataset = FashionMNIST(mode='test', transform=transform)# 模型定义model = paddle.Model(ResNet(10))# 设置训练模型所需的optimizer, loss, metricmodel.prepare(
    paddle.optimizer.Adam(learning_rate=0.001, parameters=model.parameters()),
    paddle.nn.CrossEntropyLoss(),
    paddle.metric.Accuracy(topk=(1, 5)))# 启动训练、评估model.fit(train_dataset, val_dataset, epochs=2, batch_size=64, log_freq=100)

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The loss value printed in the log is the current step, and the metric is the average value of previous step.
Epoch 1/2
step 100/938 - loss: 0.3613 - acc_top1: 0.7612 - acc_top5: 0.9820 - 3s/step
step 200/938 - loss: 0.4060 - acc_top1: 0.7972 - acc_top5: 0.9881 - 3s/step
step 300/938 - loss: 0.3635 - acc_top1: 0.8171 - acc_top5: 0.9908 - 3s/step
step 400/938 - loss: 0.3369 - acc_top1: 0.8292 - acc_top5: 0.9926 - 3s/step
step 500/938 - loss: 0.2733 - acc_top1: 0.8390 - acc_top5: 0.9937 - 3s/step
step 600/938 - loss: 0.1964 - acc_top1: 0.8469 - acc_top5: 0.9943 - 3s/step

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