Feature/transpiler split tensor to multiple pservers

ErrorClip test on MNIST

add a brief introduction of MKL-DNN work in root README.md

Enhance print_op.

* fix while_grad_op first step loss lod problem

* optimize code

* "unified operators"

* "add CUDNN register"

* "add use cudnn attribute"

* "add attribute"

* "test conv tranpose op"

* "remove duplicated attr"

* "fix op test"

* "add attribute to set cudnn"

* "add more log"

* "need layout op register support"

* "add more log"

* "change GetExpectedKernelType "

* "fix Get attr in conv_op"

* "fix CI"

* "fix tests"

* "removed kernel priority fallback"

* "fix CI"

* "fix stack pointer bug"

* "refine buggy interface"

* "add const cast to save life"

* "fix get_output_with_grad"

* "fix op test with dataformat"

* ""fix pooling

* "fix pooling test"

* "fix CI"

* "fix with_gpu error"

* "add transform needed functional check"

* "fix unpack list error"

* "comment out parallel.do temporary"

* "fix CI"

* "fix compile doc error"

* "make threshold larger"

Fix an error of the doc for how to write a new op.

add wrapper for element-wise math operator.

* update code comments.

* update the comments.

* follow comments.

add how to update whl in build_from_source.md

enbale auto set env of V1 on Mac

fix display error of C-API doc.

* add GetLoD for debug

* add LoDToString

* optimize if

* typo

* add lod_tensor to operator's dependency

Adding the rst file to render IO under Fluid in paddlepaddle.org

add doc for how to use C-API.

assign_value operator

* add cluster training bencharmk design

* update by comment

* update by comment