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

move Fluid API doc/code out of V2 API doc/code

v2 API doc overview

* fix nccl version

* enable nccl test

* Generated functions can take `*args` as inputs.

Change BN's 'mean' and 'variance' from persistable variable to Parameter

Fix compare op

add an introduction of API

Moving unique_name to python

* Also follow comments, change prefix to key

Test Parallel.Do and DynRNN

* Add reset and guard to unique_name

* framework.py enhancement

* polish

* clean up

* enforce the inputs of Operator __init__ of type Variable

* python2 assert

* reverse assert

* Add block.fwd_block_id

* fix bug in memory optimization transpiler

* Change DFS to BFS

* Add comments

Add single node tensorflow benchmark

Fix fluid dist benchmark document errors