Fix pool op doc and class declarations

fix Compile error [fatal error: boost/range/adaptor/reversed.hpp]

Simplify backward when inserting a sum operator to accumulate all duplicated variables

Add vol2col functor

* implementing softplus

* small fix

* small fix

* small fix

* small fix

Executor interface design and implementation

* Design doc of SelectedRows

* Follow comments

* Update protobuf message

* Follow comments, seperate LoDTensorDesc and SelectedRows Desc

* init optimizer design

* fix index

* optimize the interface

* add a link to python_api.md

* optimize the code of Optimizer

* Implemented the hardShrink activation

* Fixing the unit test

* Add numerically-stable logsigmoid activation

* Add softshrink operator

* Adjust relative tolerance for grad-check

* Address review comments

API of GAN

Feature/parameter