1. add gather function 2. add cross entropy forward computation and backward computation 3. code optimization 4. merge with xiaotong-working branch

1. add dropout function 2. add some function in tensor/core/math/unary file 3. merge with xiaotong-working branch

1. add macro to implement unary function 2. add sub and div function 3. merge code with the latest branch of xiaotong-working

Merge branch 'xiaotong-working' of 47.105.50.196:NiuTrans/NiuTrans.Tensor into xiaotong-working

# Conflicts:
#	.gitignore