Branch doc fixed some outdated contents in Contribute Documentations

* fix mac build error

* remove unnecessary build graph

* fix nccl op unit test

* fix build error

* format code

* refine nccl related unit test

* fix build error

* add setGPUData

* clean up

* follow comments

* rm test_nccl.cu

* follow comment

* rm wait

* add is_local paramter description

* update

* update by comment

* add sparse update section in fluid dist doc

* update by comment

* update

* update by comment

"add develop standard"

Extract Prepare from Executor

Refine the profile codes for inference.

* add memopt level

* add opt level for image classification demo

* clean code

* add delete op

* clean code

* test machine translation demo

* clean code

* clean code

* skip fill constant with force cpu

* clean code

* clean code

* refine code

* clean code

* fix bug

Add GPU compute capability check for float16 math function test

* Clipping bbox in the mAP evaluator calculation.

* Fix bug in detection_output and mAP calculation in SSD.

* Fix bug in detection.py.

* Fix bug in test_detection_map_op.py.

a little optimize of optimizer

Add ElementwiseOpInferVarType for Elementwise_op

Fix a potential bug in the c++ reader

Refine doc

Fix doc link errors

* MKLDNN pool2d OP kernel added

* conv2d and pool2d MKLDNN kernels renamed

* MKLDNN conv2d kernel refactoring

Basic double buffer for cpp reader

* fix mac build error

* override GetExpectedKernelType for cast op

* fix typo

* add cuda unittest

Fix sparse update memory error for distributed training

Update getstarted doc