* remove nccl dep when the number of GPU is 1
test=develop

* fix save/load in Fleet
* add UT framework of Fleet

test=develop

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)

* test=develop, add ocr in dygraph test

* test=develop, add cudnn determinist

* test=develop, remove useless code

* test=develop, fix cmake error

test=develop

* test=develop, add gradient sort backward strategy

* test=develop, fix test by add FLAGS_cudnn_deterministic on new tests

* remove unused FLAGS_warpctc_dir

test=develop

* remove FLAGS_warpctc_dir

test=develop

* test=develop

* test=deelop

implement dygraph.parallel.DataParallel to hook reduce op.

Support backward of backward for Relu and add a new gradient checker by comparing theoretical and numerical Jacobian. (#16862)

* Support backward of backward and a new gradient checker
* Rename decorators.py to decorator_helper.py, since Python on Windows CI has decorators package.

1. Add ReluDoubleGradMaker when register relu_grad.
2. Add a new gradient checker by comparing theoretical and numerical Jacobian.  Check double gradients by double_grad_check.

* move gc test to op_test
test=develop

* Revert "move gc test to op_test"

This reverts commit cf15da65.

* enable gc test in some ops
test=develop

* add parallel build script to ci test=develop
* 1. classify the test case as single card/two cards/multiple cards type
   2. run test case according to the run type

* enable recurrent op test=develop

test=develop

* add unit test for fuse_opt_ops
test=develop

* add seresnet unittest test=develop

* add dropout layer test=develop

* fix ci test=develop

* fix comment test=develop

* fix comment test=develop

* fix ci test=develop

* fix ci test=develop

* fix ci

* fix module name test=develop

* run imperative serenext unit test serially test=develop

test=develop

* add resnet nccl2 test=develop

* test dist train test=develop

* update test=develop

* increase timeout test=develop

* test on CI env test=develop

test=develop

test=develop

* remove est_image_classification_resnet for mac
test=develop

* increate the timeout
test=develop

* chang the timeout of test_pe_resnet
test=develop

* follow comment
test=develop

test=develop