Enforce add more helpers

enable defining sub-sequence data in test layer gradients.

Disable MKL and OpenMP by default

Initialize Gradient Checker

Save log prob

cpplint code style for *.h and *.cu

Add unittest for GradOpBuilder

Simplify building process of gradient operators

* 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

Move pybind.cc/tensor_bind.h to paddle::framework

Add @EMPTY@ to default no_grad_vars in `Backward`

Fix CPU compile