* move net_op to operators

* 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

Fix #3171

* Make interface of Operator to `const Scope&`

* use operator context

* optimize code

* update net infershape

* update InferShape

* disable override InferShape(scope) in OperatorBase

* change InferShapeImpl to InferShape

* add template to OperatorContext Input/Output

* merge Input InputVar, Output OutputVar

* change Inputs to MultiInput

* fix conflict

* fix MultiInput bugs and add unit test

* rename KernelContext to ExecutionContext

* clean code

* change InferShape to protected

* fix template bug

* refine code

* use InputVar instead of Input<Variable>

* typo

* optimize code