Merge branch 'refine_elementwise_add' of https://github.com/sneaxiy/Paddle into refine_elementwise_add

Add unit test for async transpile

filter redundant output

Add CUB Library support in Paddle

optimize vlog before and after op run, move into op.run

Add memory usage estimate API 

fix mac compile error

Fix bug when memopt optimize loss.grad and use ParallelExecutor

Fix trt output name map

* Add Preface

* Add demo code

* Save file

* Refine code

* seems can work

* use elementwise strategy

* Use ElementwiseComputeEx

* Add comments

* extract functions from operator

* Refine code

* Follow comment

* code refine

* follow comments

* follow comments

add WITH_INFERENCE flag

dist lookup table only support sgd

* polish dist se resnext ut

* update

* update

* update

* avoid cpu initializer differ

* change to use executor for now

* update by comment

* remove lr decay use para exe, should fix para exe bug later

* update by comment

* fix the order of sum

* add doc

* check whether need to copy

* follow comments

Do not set loss@Grad as persistable

Merge branch 'develop' of https://github.com/PaddlePaddle/Paddle into dist-lookup-table-only-support-sgd