add doc for how to use C-API.

assign_value operator

* add cluster training bencharmk design

* update by comment

* update by comment

Fix the documentation for elementwise op in fluid layers

Add book demo distributed fit a line

Book demo understand sentiment distributed version

Add distributed label semantic role book chapter

* Import content from the buildtools repo

* Renmae Dockerfile-x86_64 into Dockerfile.x64

* yapf

* Remove subdirectory root

We need this operator to assign value to a tensor and the values are stored in the program so that they can be used independent of python.

Feature/parallel for unittest

Feature/sync wait

Extend return value for layer functions

Make users can access parameters of layers and their gradients.

* add update graph method for memory optimization transpiler to avoid rebuild graph everytime

* clean code

* reset var desc if hit cache

Adversarial sample

Add sequence erase op

Async GRPC sendrecv

fix ds2 issue

auto set openblas env