解释一下这段代码import pdb import tensorflow as tf from matplotlib import pyplot as plt import numpy as np import os from tensorflow.keras import Model from tensorflow.keras.layers import Conv2D,BatchNormalization,Activation,MaxPool2D,Dense,Dropout,Flatten,GlobalAveragePooling2D np.set_printoptions(threshold=np.inf) 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

这段代码实现了一个ResNet18的模型。ResNet是深度学习中非常著名的神经网络模型之一，它的主要贡献在于解决了深度神经网络中的梯度消失问题，使得神经网络可以更深更复杂。ResNet18是ResNet的一个较小规模的版本，包含了18个卷积层和全连接层。这个模型的输入是一张图片，输出是一个41维的向量，用于表示图片属于41个不同类别中的哪一个。

在代码中，首先定义了一个ResnetBlock类，用于组成ResNet18中的基本模块。每个ResnetBlock包含两个卷积层和一个残差连接，其中第一个卷积层的输出经过BatchNormalization和ReLU激活函数后作为第二个卷积层的输入，第二个卷积层的输出经过BatchNormalization后和残差连接相加后再经过ReLU激活函数。如果需要进行降采样，则在残差连接中添加一个卷积层。

接着定义了一个ResNet18类，它包含了一个初始的卷积层、一系列ResnetBlock和全局平均池化层、一个全连接层。在ResNet18的构建过程中，根据传入的block_list参数的不同，逐步增加ResnetBlock的数量和输出通道数。在每个ResnetBlock中，如果需要进行降采样，则会在第一个卷积层中设置strides=2，否则为1。最后，经过全局平均池化层和全连接层，输出最终的预测结果。

在调用模型时，首先将输入图片经过初始的卷积层、BatchNormalization和ReLU激活函数，再经过一系列ResnetBlock，最后进行全局平均池化和全连接层的计算，得到预测结果。