* cherry pick test=develop

* cleanup test=develop test=release/1.6

* cleanup test=develop test=release/1.6

Split the unittest test_dist_mmist into multiple unittests (test_dist_mnist, test_dist_mnist_nccl and test_dist_mnist_lars) to avoid timeout (#17707)

* wip allreduce in op

* wip

* wip

* wip

* wip adding test

* wip for conflict with mp mode

* fix tests test=develop

* fix cpu build test=develop

* fix travis clang format test=develop

* fix cpu build test=develop

* update api.spec test=develop

* delete comment test=develop

* fix cpplint test=develop

* fix test=develop

* follow comment test=develop

* add file test=develop

* fix build test=develop

* update test=develop

* to be compatible with sync_bn, and fix mp mode in develop test=develop

* fix nccl dist test acc test=develop

* fix test=develop

* add nccl2 dist tests test=develop

* fix dist_base test=develop

* fix tests test=develop

* fix test on mac test=develop

* wip

* add ref_by_trainer_id op

* ready to test

* fix ref inputs

* refine rpc_op_handle

* fix merge bug

* fix nccl2 lars dist support

* put lars in momentum op

* add tests lars

* fix ci

* fix cpu kernel

* soft warning

* remove lars in test_recognize_digits.py

* move to another op

* add file

* update api.spec test=develop

* update test=develop

* fix api.spec test=develop

* wip

* wip, finish grad merge ops

* wip, finish graph build

* wip test running

* work on 1 gpu

* workable version

* update

* fix tests

* fuse broadcast op

* fix compile failed

* refine

* add batch merge test mnist

* fix CI test=develop

* fix build

* use independent bn params for batch merge test=develop

* update api.spec

* follow comments and for test

* wip

* refine tests test=develop

* follow comments test=develop

* remove startup bn modify test=develop

* follow comments test=develop

* fix merge test=develop

* fix graph num hang test=develop

* re-enable tests test=develop

* re-enable graph num check test=develop

* fix multi device pass role check test=develop

* disable async dist test

* update test=develop

* add dist ut for text_classification

* add dist ut for text_classification

* add simnet bow unittest

* add dist ut for simnet bow

* add trainning data url for simnet bow

* add trainning data url for simnet bow

* modify simnet test_reader to train reader

* add test_dist_ctr

* test_dist_ctr can run now

* dense update is good

* add unit test for selected rows

* debug unit test

* fix dist sparse update problem

* Constant args at init

* optimize code

* simnet optimize

* fix DebugStringEx

* optimize sum_op.h

* add ScaleOpVarTypeInference

* clean code

* fix test_dist_transpiler.py

* code optimize

* modify delta

* fix sparse update bug

* dist test use one cpu

* update some data

* remove unused code

* add use cuda config

* unit test fix

* unit test fix

* unit test fix

* unit test fix

* dist_word2vec use CPU

* unit test fix

* unit test fix

* code clean

* code clean

* merge develop

* api spec update

* Revert: api spec update

* replace simnet data with fake

* replace simnet data with fake

* update dim

* add batch auc

* code clean

* code clean

* modify print to stderr

* update simnet delta -> 1e-5

* update RUN_STEP

* add use_reader_alloc

* add use_reader_alloc

* add use_reader_alloc

* modify delta

* add use_reader_alloc

* fix stderr write

* python3 compatibility

test=develop

* python3 compatibility, test=develop

* Update dist_text_classification.py

* test=develop

* wip

* clean up

* should fix running with memopt

* add ut

* mark lr schedule op role

* hide lr_schedule_guard

* use op_role_var instead of ufind

* unify dist test name

* wip for py3 support

* fix var deref

* fix python3 mem_opt order

* remove comments

* fix dist train reduce mode

* fix previous fix

* add test for dist and memopt

* update transformer too

* add async dist tests

* update delta

* fix transformer test

* refine rmsprop transpile

* update

* fix dist seresnet