def __init__(self, block_list, initial_filters=64): # block_list表示每个block有几个卷积层 super(ResNet18, self).__init__() self.num_blocks = len(block_list) # 共有几个block self.block_list = block_list self.out_filters = initial_filters self.c1 = layers.Conv2D(self.out_filters, (3, 3), strides=1, padding='same', use_bias=False) self.b1 = layers.BatchNormalization() self.a1 = layers.Activation('relu') self.blocks = tf.keras.models.Sequential() # 构建ResNet网络结构 for block_id in range(len(block_list)): # 第几个resnet block for layer_id in range(block_list[block_id]): # 第几个卷积层 if block_id != 0 and layer_id == 0: # 对除第一个block以外的每个block的输入进行下采样 block = ResnetBlock(self.out_filters, strides=2, residual_path=True) else: block = ResnetBlock(self.out_filters, residual_path=False) self.blocks.add(block) # 将构建好的block加入resnet self.out_filters *= 2 # 下一个block的卷积核数是上一个block的2倍 self.p1 = tf.keras.layers.GlobalAveragePooling2D() self.f1 = tf.keras.layers.Dense(10, activation='softmax', kernel_regularizer=tf.keras.regularizers.l2())解释代码

这段代码是定义了一个 ResNet18 神经网络模型，它采用了残差网络（ResNet）的结构。ResNet18 模型由一系列 ResnetBlock 组成，每个 ResnetBlock 包含若干个卷积层。其中，第一个 ResnetBlock 的输入经过一个卷积层后直接进入该 ResnetBlock，而后续的每个 ResnetBlock 的输入则需要通过一个下采样的卷积层进行降维，以便与该 ResnetBlock 中的输出进行相加。最终，模型的输出经过全局平均池化后，再经过一个全连接层输出预测结果。同时，模型中还使用了批归一化（BatchNormalization）和 L2 正则化（kernel_regularizer=tf.keras.regularizers.l2()）等技术来提高模型的泛化能力。

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解释一下这段代码：class ResnetBlock(Model): def __init__(self, filters, strides=1,residual_path=False): super(ResnetBlock, self).__init__() self.filters = filters self.strides = strides self.residual_path = residual_path self.c1 = Conv2D(filters, (3, 3), strides=strides, padding='same', use_bias=False) self.b1 = BatchNormalization() self.a1 = Activation('relu') self.c2 = Conv2D(filters, (3, 3), strides=1, padding='same', use_bias=False) self.b2 = BatchNormalization() if residual_path: self.down_c1 = Conv2D(filters, (1, 1),strides=strides, padding='same', use_bias=False) self.down_b1 = BatchNormalization() self.a2 = Activation('relu') def call(self, inputs): residual = inputs x = self.c1(inputs) x = self.b1(x) x = self.a1(x) x = self.c2(x) y = self.b2(x) if self.residual_path: residual = self.down_c1(inputs) residual = self.down_b1(residual) out = self.a2(y + residual) return out class ResNet18(Model): def __init__(self, block_list, initial_filters=64): super(ResNet18, self).__init__() self.num_blocks = len(block_list) self.block_list = block_list self.out_filters = initial_filters self.c1 = Conv2D(self.out_filters, (3, 3), strides=1, padding='same', use_bias=False, kernel_initializer='he_normal') self.b1 = BatchNormalization() self.a1 = Activation('relu') self.blocks = tf.keras.models.Sequential() for block_id in range(len(block_list)): for layer_id in range(block_list[block_id]): if block_id != 0 and layer_id == 0: block = ResnetBlock(self.out_filters, strides=2, residual_path=True) else: block = ResnetBlock(self.out_filters, residual_path=False) self.blocks.add(block) self.out_filters *= 2 self.p1 = tf.keras.layers.GlobalAveragePooling2D() self.f1 = tf.keras.layers.Dense(41, activation='tanh') def call(self, inputs): x = self.c1(inputs) x = self.b1(x) x = self.a1(x) x = self.blocks(x) x = self.p1(x) y = self.f1(x) return y

这段代码定义了两个类：ResnetBlock和ResNet18。ResnetBlock是ResNet18的基本模块，用于构建ResNet18的主体结构。ResnetBlock包含两个卷积层和批归一化层，以及一个残差路径。ResNet18是一个由ResnetBlock组成的深度卷积神经网络，包含多个ResnetBlock，每个ResnetBlock中包含多个卷积层和批归一化层。在ResNet18的定义中，先定义了第一个卷积层、批归一化层和激活层，然后通过循环创建多个ResnetBlock，并在每个ResnetBlock中根据需要定义残差路径，最后通过全局平均池化层和全连接层输出预测结果。

详细解释一下这段代码：class ResnetBlock(Model): def init(self, filters, strides=1,residual_path=False): super(ResnetBlock, self).init() self.filters = filters self.strides = strides self.residual_path = residual_path self.c1 = Conv2D(filters, (3, 3), strides=strides, padding='same', use_bias=False) self.b1 = BatchNormalization() self.a1 = Activation('relu') self.c2 = Conv2D(filters, (3, 3), strides=1, padding='same', use_bias=False) self.b2 = BatchNormalization() if residual_path: self.down_c1 = Conv2D(filters, (1, 1),strides=strides, padding='same', use_bias=False) self.down_b1 = BatchNormalization() self.a2 = Activation('relu') def call(self, inputs): residual = inputs x = self.c1(inputs) x = self.b1(x) x = self.a1(x) x = self.c2(x) y = self.b2(x) if self.residual_path: residual = self.down_c1(inputs) residual = self.down_b1(residual) out = self.a2(y + residual) return out class ResNet18(Model): def init(self, block_list, initial_filters=64): super(ResNet18, self).init() self.num_blocks = len(block_list) self.block_list = block_list self.out_filters = initial_filters self.c1 = Conv2D(self.out_filters, (3, 3), strides=1, padding='same', use_bias=False, kernel_initializer='he_normal') self.b1 = BatchNormalization() self.a1 = Activation('relu') self.blocks = tf.keras.models.Sequential() for block_id in range(len(block_list)): for layer_id in range(block_list[block_id]): if block_id != 0 and layer_id == 0: block = ResnetBlock(self.out_filters, strides=2, residual_path=True) else: block = ResnetBlock(self.out_filters, residual_path=False) self.blocks.add(block) self.out_filters *= 2 self.p1 = tf.keras.layers.GlobalAveragePooling2D() self.f1 = tf.keras.layers.Dense(41, activation='tanh') def call(self, inputs): x = self.c1(inputs) x = self.b1(x) x = self.a1(x) x = self.blocks(x) x = self.p1(x) y = self.f1(x) return y

这段代码定义了两个类：ResnetBlock 和 ResNet18，用于实现 ResNet18 模型。

ResnetBlock 类定义了一个残差块，包含两个卷积层和一个残差连接。其中，第一个卷积层 c1 用于提取特征，第二个卷积层 c2 用于将特征进行降维，BatchNormalization 层用于加速训练，Activation 层使用 ReLU 激活函数对卷积结果进行非线性变换。如果 residual_path 参数为 True，则使用下采样卷积层 down_c1 和下采样 BatchNormalization 层 down_b1 对输入进行下采样，然后再将下采样结果与卷积结果相加，即为残差连接。最后使用 Activation 层进行激活。

ResNet18 类定义了一个 ResNet18 模型，包含多个 ResnetBlock 残差块和全局平均池化层和一个全连接层。使用 Conv2d 卷积层提取特征，再使用 BatchNormalization 层加速训练，使用 Activation 层进行激活，然后将结果经过多个 ResnetBlock 残差块进行特征提取和降维，最后使用 GlobalAveragePooling2D 层进行池化操作，将结果送入全连接层进行分类，输出结果。其中，block_list 参数表示每个残差块包含的 ResnetBlock 个数，initial_filters 参数表示初始通道数。如果不是第一个残差块并且是第一个 ResnetBlock，则使用 strides=2 进行下采样。