* add memory optimization transpiler demo

* add memory benchmark compile option

* add gflags instead of macro

* refine code

* "fix decode bug"

* "follow commnet"

* "fix error"

* "fix hook bug"

* fix based comment

* fix copyright

* fix based on comment

Add distribution implement of image classification

* registery is only used by layers
* Rename it to layer_function_generator

Users can use `a+b`, `a*10`.

* add split selected rows op

* update comment

* add grad check

* registry cuda kernel

* fix ci failed

update doc and dist test due to API change https://github.com/PaddlePaddle/Paddle/pull/7619#pullrequestreview-89656079