形状元组中的None维度指的是批处理维度，这仅表示该层可以接受任何大小的输入.例如，数据集(例如MNIST)可能具有以下形状元组(60000, 28, 28, 1)，但输入层的形状将为(None, 28, 28, 1).

The None dimension in the shape tuple refers to the batch dimension which simply means that the layer can accept input of any size. For example, a dataset (e.g. MNIST) could have the following shape tuple (60000, 28, 28, 1) but the input layer's shape would be (None, 28, 28, 1).

参考文献:

• keras-team/keras#2054
• Keras文档-核心层

• keras-team/keras#2054
• Keras Documentation - Core Layers