动手学pytorch-Batch Norm

批量归一化

1.基本概念

2.代码实现

1.基本概念

对输入的标准化(浅层模型)
处理后的任意一个特征在数据集中所有样本上的均值为0、标准差为1。
标准化处理输入数据使各个特征的分布相近
批量归一化(深度模型)
利用小批量上的均值和标准差,不断调整神经网络中间输出,从而使整个神经网络在各层的中间输出的数值更稳定。

1.1对全连接层做批量归一化

位置:全连接层中的仿射变换和激活函数之间。
全连接:
\[ \boldsymbol{x} = \boldsymbol{W\boldsymbol{u} + \boldsymbol{b}} \\ output =\phi(\boldsymbol{x}) \]

批量归一化:
\[ output=\phi(\text{BN}(\boldsymbol{x})) \]
\[ \boldsymbol{y}^{(i)} = \text{BN}(\boldsymbol{x}^{(i)}) \]
\[ \boldsymbol{\mu}_\mathcal{B} \leftarrow \frac{1}{m}\sum_{i = 1}^{m} \boldsymbol{x}^{(i)}, \]

\[ \boldsymbol{\sigma}_\mathcal{B}^2 \leftarrow \frac{1}{m} \sum_{i=1}^{m}(\boldsymbol{x}^{(i)} - \boldsymbol{\mu}_\mathcal{B})^2, \]

\[ \hat{\boldsymbol{x}}^{(i)} \leftarrow \frac{\boldsymbol{x}^{(i)} - \boldsymbol{\mu}_\mathcal{B}}{\sqrt{\boldsymbol{\sigma}_\mathcal{B}^2 + \epsilon}}, \]
这⾥ϵ > 0是个很小的常数,保证分母大于0
\[ {\boldsymbol{y}}^{(i)} \leftarrow \boldsymbol{\gamma} \odot \hat{\boldsymbol{x}}^{(i)} + \boldsymbol{\beta}. \]

引入可学习参数:拉伸参数γ和偏移参数β。若\(\boldsymbol{\gamma} = \sqrt{\boldsymbol{\sigma}_\mathcal{B}^2 + \epsilon}\)和\(\boldsymbol{\beta} = \boldsymbol{\mu}_\mathcal{B}\),批量归一化无效。

1.2对卷积层做批量归⼀化

位置:卷积计算之后、应⽤激活函数之前。
如果卷积计算输出多个通道,我们需要对这些通道的输出分别做批量归一化,且每个通道都拥有独立的拉伸和偏移参数。
计算:对单通道,batchsize=m,卷积计算输出=pxq
对该通道中m×p×q个元素同时做批量归一化,使用相同的均值和方差。

1.3预测时的批量归⼀化

训练:以batch为单位,对每个batch计算均值和方差。
预测:用移动平均估算整个训练数据集的样本均值和方差。

2.代码实现

class BatchNorm(nn.Module):
    def __init__(self, *, num_features, num_dims):
        super(BatchNorm, self).__init__()
        super(BatchNorm, self).__init__()
        if num_dims == 2:
            shape = (1, num_features) #全连接层输出神经元
        else:
            shape = (1, num_features, 1, 1)  #通道数
        # 参与求梯度和迭代的拉伸和偏移参数,分别初始化成0和1
        self.gamma = nn.Parameter(torch.ones(shape))
        self.beta = nn.Parameter(torch.zeros(shape))
        # 不参与求梯度和迭代的变量,全在内存上初始化成0
        self.moving_mean = torch.zeros(shape)
        self.moving_var = torch.zeros(shape)
        self.momentum = 0.9
    
    def forward(self, X):
        if self.moving_mean.device != X.device:
            self.moving_mean = self.moving_mean.to(X.device)
            self.moving_var = self.moving_var.to(X.device)
        # 保存更新过的moving_mean和moving_var, Module实例的traning属性默认为true, 调用.eval()后设成false
        Y, self.moving_mean, self.moving_var = self._batch_norm(self.training, 
            X, self.gamma, self.beta, self.moving_mean,
            self.moving_var, eps=1e-5, momentum=self.momentum)
        return Y

    def _batch_norm(self, is_training, X, gamma, beta, moving_mean, moving_var, eps, momentum):
        if not is_training:
            # 如果是在预测模式下,直接使用传入的移动平均所得的均值和方差
            X_hat = (X - moving_mean) / torch.sqrt(moving_var + eps)
        else:
            assert len(X.shape) in (2, 4)
            if len(X.shape) == 2:
                # 使用全连接层的情况,计算特征维上的均值和方差
                mean = X.mean(dim=0)
                var = ((X - mean) ** 2).mean(dim=0)
            else:
                # 使用二维卷积层的情况,计算通道维上(axis=1)的均值和方差。这里我们需要保持
                # X的形状以便后面可以做广播运算
                mean = X.mean(dim=0, keepdim=True).mean(dim=2, keepdim=True).mean(dim=3, keepdim=True)
                var = ((X - mean) ** 2).mean(dim=0, keepdim=True).mean(dim=2, keepdim=True).mean(dim=3, keepdim=True)
            # 训练模式下用当前的均值和方差做标准化
            X_hat = (X - mean) / torch.sqrt(var + eps)
            # 更新移动平均的均值和方差
            moving_mean = momentum * moving_mean + (1.0 - momentum) * mean
            moving_var = momentum * moving_var + (1.0 - momentum) * var
        Y = gamma * X_hat + beta  # 拉伸和偏移
        return Y, moving_mean, moving_var

带batch norm 的LeNet

class BLeNet(nn.Module):
    def __init__(self, *, channels, fig_size, num_class):
        super(BLeNet, self).__init__()
        self.conv = nn.Sequential(
            nn.Conv2d(channels, 6, 5, padding=2),
            BatchNorm(num_features=6, num_dims = 4),
            nn.Sigmoid(),
            nn.AvgPool2d(2, 2),
            nn.Conv2d(6, 16, 5),
            BatchNorm(num_features=16, num_dims = 4),
            nn.Sigmoid(),
            nn.AvgPool2d(2, 2),
        )
        ##经过卷积和池化层后的图像大小
        fig_size = (fig_size - 5 + 1 + 4 ) // 1
        fig_size = (fig_size - 2 + 2) // 2
        fig_size = (fig_size - 5 + 1) // 1
        fig_size = (fig_size - 2 + 2) // 2
        self.fc = nn.Sequential(
            nn.Flatten(),
            nn.Linear(16 * fig_size * fig_size, 120),
            BatchNorm(num_features=120, num_dims = 2),
            nn.Sigmoid(),
            nn.Linear(120, 84),
            BatchNorm(num_features=84, num_dims = 2),
            nn.Sigmoid(),
            nn.Linear(84, num_class),
        )
    def forward(self, X):
        conv_features = self.conv(X)
        output = self.fc(conv_features)
        return output
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