https://zh-v2.d2l.ai/chapter_multilayer-perceptrons/dropout.html
num_epochs, lr, batch_size = 10, 0.5, 256
loss = tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True)
train_iter, test_iter = d2l.load_data_fashion_mnist(batch_size)
trainer = tf.keras.optimizers.SGD(learning_rate=lr)
d2l.train_ch3(net, train_iter, test_iter, loss, num_epochs, trainer)
这里的训练并没有使用上面NET定义的training参数，导致改变dropout参数不起作用？