* expose peak gpu memory API to python test=develop

* add unittest for peak gpu memory monitoring test=develop

* add pybind change test=develop

* add mutex to gpu mem usage monitor test=develop

* update benchmark flag definition file test=develop

* tweak unittest for memory monitoring test=develop

* fix github issue 15267 test=develop

* fix github issue 15267 test=develop

* monitor the GPU usage during runtime

* revert allocator_facade.cc change

* comments update test=develop

test=develop

* wip multi process multi gpu dist training

* workable for p2p

* update test=develop

* change back env name test=develop

* fix alloc init

* fix cpu build test=devlop

* fix mac tests test=develop

* refine code

* refine test=develop

test=develop

test=develop

* human readable memory warns test=develop

* update test=develop

* refine test=develop

* fix build test=develop

test=develop

test=develop

Fix code to support cpplint syntax check

test=develop

- add naive executor
- fix concurrency performance issue

Use OO style to rewrite memory allocation.

* memory init

* add env

* refine anounce

* Add check for Nan

* Debug

* Add env for cc_test

* Add env for py_test and nv_test

* Remove py_test env

* Add env for py_test

* serial test_recognize_digits

* Test FLAGS_init_allocated_mem function for unit test

* Init allocated mem for op unit test

* Add env for all unit test

* Fix cpplint errors with paddle/fluid/platform/gpu_info.*

* Update

* fix gpu memory allocation policy

* refine codes

* fix code style

* follow comments

* add gflags for C++ unittest