* support sparse gradients
test=develop

test=develop

* feature/buffer_shared_inplace, test=develop

* refine code, test=develop

* fix elementwise_add op cpu inplace and sum inplace bug, test=develop

* add unittest and debug log, test=develop

* fix parallel_executor scope bug, polish code, test=develop

* fix sum op, activation op, single_in_place_inference bug, test=develop

* remove kLocalExecScopeName, test=develop

* fix unittest,test=develop

* fix out_var first version bug, test=develop

* follow comments,test=develop

* clean code of dim and place, test=develop

* fix failed unittests, test=develop

* add is_runnning in communicator, test=develop

* clean build_strategy
test=develop

* DataBalanceOpHandle has been removed
test=develop

* debug

* update build_strategy.
test=develop

* fix code bug
test=develop

test=develop

* add event for fast executor and add threads for scopebuffer executor
test=develop

Async exe support communicator

test=develop

* add record_event
test=develop

* remove csp
test=develop

Refine the Executor when the num_thread=1

* make parallel_executor support FLAGS_use_mkldnn

test=develop

* add warning when set mkldnn_enabled_op_types_ in non-mkldnn env

test=develop

* reset drop local scope counter
test=develop

test=develop

* move pass to ir

* polish code
test=develop

* fix dependency
test=develop

* add use_cuda to inplace pass,test=develop

* add test softmax_with_xe_inplace test,test=develop

* fix potential inplace bug
test=develop

* add more skip vars in mem opt pass,test=develop

* follow comment,test=develop

* follow comments,move duplicate out arg check to program->graph,test=develop

* add use_cuda to inplace pass,test=develop

* add test softmax_with_xe_inplace test,test=develop

test=develop

* fix op graph view
test=develop

* rewrite inplace pass and fix reference count pass bug
test=develop

* fix unittest failed
test=develop

* follow comments, test=develop

test=develop

test=develop

test=develop

* Add fuse momenutum ops

* speedup gc and inplace softmax_with_cross_entropy_grad
test=develop

* refine models gpu mem
Merge skip vars and warning messages of mem opt
remove relu mem opt
test=develop

* follow comments
test=develop

* fix bug of fuse optimize ops

* replace ThreadedExecutor with FastThreadedExecutor
test=develop

* Fix Travise CI
test=develop

* Test FastThreadedSSAGraphExecutor
test=develop

* refine parallel_ssa_graph_executor.cc
test=develop

* Add an option to enable the cache of expected kernel in train phase.
test=develop

* Change the default value of cache_expected_kernel to true.

test=develop

* bug-fix, test=develop

* tweak code, test=develop

* test fuse all_reduce