* Update backward appending stragety to support double backward and fix some bug. (#18104)

* Update backward.py:
     - If there is no input grad var in all outputs of previous ops, do not append this op into graph.
     - Only apply this stragety when double backward.
* Update some double backward op.
* Update sum_op to judge whether a tensor is empty by numel or IsInitialized().

* fix dygraph mem leak, test=release/1.5

* polish msg, test=release/1.5

* add gradient clip in minimize; test=develop

* fix bug; test=develop

* fix format; test=develop

* move new grad clip to dygraph/grad_clip.py; test=develop

* fix lr decay and grad clip test; test=develop

* seperate dygraph grad clip; test=develop

* fix grad clip test; develop

* fix api spec bug; test=develop

* add blank line, test=develop,test=document_preview

to fix format problem

* optimize communicator flag

* change flags in init py test=develop

* remove unused FLAGS_warpctc_dir

test=develop

* remove FLAGS_warpctc_dir

test=develop

* remove async executor python api
test=develop

* remove test_async_executor.py
add executor train_from_dataset demo
test=develop

* fix import bug
test=develop

* fix cudnn batch norm accuracy test=develop

* fix cudnn batch norm accuracy test=develop

* disable failed test for later fix test=develop

test=develop

test=develop

* test=develop, add install check

* test=develop, add install check scripts

* test=develop, refine language

* test=develop, add api spec

* test=develop, change cdn to bj to pass ci

test=develop

modify allocator strategy
remove changes of legacy buddy_allocator
test=develop

* fuse all_reduce
test=develop

* add fuse_parameter_groups_size
test=develop

* Polish code
test=develop

* Fix travis-ci
test=develop

* Add SetGroupAccordingToLayers and SetGroupAccordingToGroupSize
test=develop

* Add SetGroupAccordingToMemorySize
test=develop

* fix multi_devices_graph
test=develop

* reset params_grads
test=develop

* Polish code
test=develop

test=develop

* remove concast_cast and refactor code test=develop

* reduce flag use test=develop

test=develop

* add ir memory optimize test case for IfElse op, test=develop

* fix some unitttest failure by force using the python memory_optimize, test=develop

* tweak comments, test=develop

* fix unittest, test=develop

* fix unittest, test=develop

* add ir memory optimize test case for IfElse op, test=develop

* fix some unitttest failure by force using the python memory_optimize, test=develop

* tweak comments, test=develop

* fix unittest, test=develop

* fix unittest, test=develop

test=develop

test=develop

* refine profiler && add runtime tracer

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* test=develop

* fix bug && test=develop

* add thread id map && test=develop

* test=develop

* testing

* bug fix

* remove cuda event && refine code && test=develop

* test=develop

* test=develop

* test=develop

* fix windows temp file && test=develop

* test=develop

* fix windows bug && test=develop

* fix start up issue && test=develop

* code polish &&  test=develop

* remove unused code && test=develop

* add some cupti cbid && test=develop

* add FLAGS_multiple_of_cupti_buffer_size && test=develop

* fix compile error && test=develop

* add keyword && test=develop

* fix && test=develop

* code polish && test=develop

* add launch mp distributed mode module test=develop

* delete unused file test=develop

* refine usage test=develop

* refine usage test=develop

* move distributed package test=develop

* add to whl package test=develop

test=develop

test=develop

test=develop
This reverts commit 358e657f.

test=develop
This reverts commit 064512aa.

* remove workspace_handle in conv2d_cudnn
test=develop

* remove workspace_handle
test=develop

* fix bug
test=develop

* make test_conv2d_op SERIAL
test=develop

* save memory in conv_cudnn
test=develop

* enhance thread safety
test=develop

* enhance temporary allocator
test=develop

* Add excess fraction
test=develop

* follow comments
test=develop

* fix bug and code refine
test=develop

* fix memory size check
test=develop

* rename reuse_tmp_allocation_excess_fraction
test=develop