Improve test_parallel_executor.

* remove net op and cond_op

* fix cpplint

* fix dependency

* delete backward_test; fix compile

* disable batch_norm backward

* rm test_net.py

* make batchnorm test independent of backward.cc

* make test_layer_norm_op independent of backward.cc

* make test_layer_norm_op independent of backward.cc

* delete unused code

* clean up

* Fix cpplint errors, round2

* Fix pointer issue

Disable unstable unittest

remove unused nccl.cmake

[Feature] Enable multi gpu distributed training of fluid

Fix api docs display error for fluid Initializer

 Add contents for manully build documentation(cn version)

* "add metrics"

* "add fluid metrics"

* "add import guards"

* "show warnings"

* "add demo"

* "fix ci"

* "add some details"

* "fix cci"

* "add demo Python"

* "add metrics"

Modify readers to fit the parallel executor

Make cuda_helper.h Pass cpplint

k8s dist train for en

add remove_op, remove_var in Python end

* fix gemm error for cuda 7.5

* fix version number

Fix lost of LoD while splitting tensor in parallel executor.

* Update compile_paddle_lib_en.md

Fix https://github.com/PaddlePaddle/Paddle/issues/8916

* Update compile_paddle_lib_en.md