在进行mnist识别时，https://www.charleychai.com/blogs/2018/ai/NN/lenet.html中，
X_train = X_train.reshape(-1, 28, 28, 1)
X_test = X_test.reshape(-1, 28, 28, 1)
为什么要设置最后一维为1呢？
可以查看Conv2D中对input_shape的说明，https://keras.io/layers/convolutional/
Input shape
4D tensor with shape: (batch, channels, rows, cols) if data_format is "channels_first" or 4D tensor with shape: (batch, rows, cols, channels) if data_format is "channels_last".
对于代码中，-1是指batch，共有多少个样本，28分别指rows和cols，最后一个是指通道数，由于不是RGB，所以是1。