* polish backward api doc test=develop, test=document_preview,
       test=document_fix

* polish backward api doc test=develop, test=document_preview, test=document_fix

* no_grad supports set of Variable test=develop, test=document_preview

* polish sample code of append_backward test=develop, test=document_preview

* modify assert into Raise TypeError test=develop,test=document_preview

* fix unittest failed test=develop

* rm useless file test=develop

* polish en doc test=develop

* polish code of no_grad_set test=develop

* polish code of no_grad_set test=develop

* add skip_check_grad_ci of var_conv_2d test=develop

* modify check_shape_white_list test=develop

cmake选项中添加了WITH_NCCL，显示指定是否编译NCCL的部分代码，WITH_NCCL默认打开，但如果WITH_GPU为OFF，则关闭WITH_NCCL

添加了PADDLE_WITH_NCCL定义

单机单卡能够关闭NCCL编译，多卡的话需要默认打开NCCL，如果关闭NCCL，则只能使用单卡
Co-authored-by: N石晓伟 <39303645+Shixiaowei02@users.noreply.github.com>

* add hdfs ls retry time and sleep time, fix save inference
* test=develop

* add int16 support, test=develop

* add test, test=develop

* fix typo, test=develop

* fix dtype error in slice, test=develop

* fix chain doc, test=develop, test=document_preview

* fix bug with half communicator

* update the ut precision of pad pad2d pad_constant_like from fp32 to fp64, test=develop

* add GeneralRoleMaker which is for general usage
* test=develop

* add no_grad_set value check in op_test, test=develop

* update ops list, test=develop

* test=develop, fix geo Send & Init

test=develop

* remove skip_check in test_activation_mkldnn_op, test=develop

* polish backward prune, test=develop

* fix control flow op bug, test=develop

* add some unittests, test=develop

* fix unittest args, test=develop

* follow huihuang's comments, test=develop

* fix dataloader early reset bug, test=develop

* change implementation, test=develop

* fix ut, test=develop

fix timeout of test_dist_fleet_geo

* Implement a common python unittest to test the ir passes.
test=develop

* Save the results in np.array and support to startup on CPU.
test=develop

* Fix the unittest.
test=develop

* Add check_program to check whether the optimized program is different from the origin one.
test=develop

* Remove the inferface all_ops.
test=develop

* Add exception test in pass_test.
test=develop

* fix circular dependent

* try import layers.nn from dygraph
test=develop

* allow sublayer or param shadow attrs

* add unittest
test=develop

* change remove fn name
test=develop

* add half_async in the communicator
* fix DistributedStrategy

* add learning rate api; test=develop

* fix uni test converage; test=develop

* fix travis ci error; test=develop

* fix comment; test=develop

* fix example error; test=develop

* polish the api description, test=develop
Co-authored-by: Nzhongpu <2013000149@qq.com>

* Fixed warpctc, test=develop

* Set lod level of sequence_unpad's output to 1 in compile time

test=develop

* fix the en doc and example code of warpctc, test=develop, test=document_fix

* fix test_layers compare static graph and dygraph result; test=develop

* fix test_layers random error; test=develop

* add multiprocess for dygraph data loader, test=develop

* polish code & add safe gurad, test=develop

* refactor dygraph dataloader & add signal handler, test=develop

* fix member initializer compile error on ci, test=develop

* fix member initializer compile error one more, test=develop

* remove useless config, test=develop

* skip windows incompatible problem, test=develop

* add unittest for coverage, test=coverage

* add more exception unittest case, test=develop

* deal with signal handler coverage, test=develop

* polish code & add signal handler tests, test=develop

* deal with coverage ci problem, test=develop

* split data loader test & coverage ci fix, test=develop

* remove test_imperative_data_loader_with_exception, test=develop

* remove singal process except test case, test=develop

* add exception tests again & remove sample list test, test=develop

* split normal and exception unittests to diff class, test=develop

* polish doc for use_multiprocess effect in static mode, test=develop