https://www.cnblogs.com/ranjiewen/p/9278631.html
https://www.cnblogs.com/shixiangwan/p/7536369.html
note:
depthwise_conv2d is not a depthwise separable convolution
Depthwise convolution (depthwise convolution) followed by 1 * 1 convolution is
Depthwise Separable Convolution ( tf.nn.separable_conv2d )