refine MKLDNNLayer logical: move forward and backward to MKLDNNLayer and remove copyOutputInfoToOtherDevice

add concat op with CPU kernel

It is buggy in some Linux because the unique_ptr will be free however
the std::string trying to use that char*.

Moreover, it is no need to demangle for error log by Paddle.
Just use `c++filt` or other shell utilities to do this.

1. Rename variables by Google style.
2. Add more test cases.
3. Add more detail and meaningful comments.
4. Change type of "padding" to vector<int>