Refine buffer receive

Add type `Reader` for `VarDesc`

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into dev_make_VarDesc_supporting_multiple_tensor

Fix warnings in multiclass_nms_op.cc.

* Update csp.md

* Update csp.md

* Update csp.md

helper functions fetch_var and get_var

* Add proposed fluid syntax for select statement in Fluid's implementation of CSP

* Fix Typo

* Adding send and recv in design doc

* fix typo

* fixed code

* Adding threading

Refine prior_box_op

add independent sphinx tree for api

Add a new type `Reader` for `VarDesc`, which can holds more than one
LoDTensor.

Add `make clean` in docker/build.sh

remove duplicated mobile index

Fix the error when sorted_key is none in profiler

* refine buffer channel

*  refine Receive and Send

* follow comments

* Adding more receivers less senders

* Added more receivers less senders

* Added more send

* Updated comment

* Fixed code style

* Fixing review comments

Buffered channel should notify both condition variables on close

Add depthwise conv op gpu

simplify shape inference code

Refine elementwise_op