* add support_gpu

* fix allclose

* fix name error and symplify code

* finish forward debug

* init cross entropy graident

* add crossentropy grad op

* remove details

* fix static compile

Make ci pass

Currently use `Operator("fc", X="x", W='w1', B='b1')` as operator
creation method.

Fix #3198

Rename operator.py to op.py because it is conflict with protobuf

* init softmax grad op

* add compute code

* export Backward to python

* update test ,export op.type to python

* update python test, fix compute bug

* update unit test

* use eigen

* optimize eigen code

* add gpu test

* register softmax_grad GPU kernel and fix test bug

* typo

* follow comments

* export Backward to python

Add get_numeric_gradient API and its unittest.

* add start_record interface

* call master client in reader

* update

* add demo code in comments

* update comments

* delete unittest for recordio reader

* Make interface of Operator to `const Scope&`