2. Make SwitchOrderLayer support for softmax activation
3. Fix bugs

1. Add switch function for switching image dimensions order
2. Add CpuMatrix::backwardSoftmax function
3. Add pixel softmax layer, python wrapper and grad_test

Add the use_nnpack parameter in ExpandConvLayer, so that the convolution calculation can be switched to the NNPACK function.

Fix #2567

Modify the arguments description of ConvFunctionBase. And add the definition of backward input and backward filter function.