cherry pick pr: 17904， dygraph Layers doc

* add api commet; test=develop

* fix fc dtype bug; test=develop

* remove float32 in default parameter; test=develop

* fix exmpale bug; test=develop

* fix build once; test=develop

* fix num_chanels bug; test=develop

* fix install check failed bug; test=develop

* WIP

* WIP

* test=develop, add api doc and example code for dygraph

* test=develop, refine api

* test=develop, fix bug when error occured on save_persistable with no optimizer

* test=develop, refine waring

* test=develop, refine example code and comments

* test=develop, add dygraph_not_support and refine ocr

* test=develop, shrink name of dygraph_not_support

* add import, test=develop

* fix fill_constant

* fix deconv

* add trainable_statist for bn in dygraph

* save optimizer related vars in dygraph

* test=develop, add optimizer save and load

* test=develop, add optimizer save and load

* test=develop, merge code and add multi-optimizer save and load

* test=develop, fix test_imperative_checkpoint

* test=develop, fix include error

* test=develop, fix include error

* test=develop, renew api spec

* test=develop, refine code

* test=develop, set default value for checkpoint

* test=develop, fix ci error

* test=develop, change API.spec and make api more readable

* test=develop, refine version and time stamp

* test=develop, add example code and refine code

* test=develop, refine doc

* test=develop, change version

* use python api to reduce python time cost, test=develop

* fix travis ci, test=develop

* fix Py_None error,test=develop

* for debug

* test=develop, memory optimize for dygraph using shared_ptr

* test=develop, fix travis ci showed error

* test=develop, fix bug for recurrent usage of varbase

* test=develop, init varbase when it need to be Add

* add no_grad decorator to dygraph, test=develop

* add unittest,test=develop

 use numpy assignment to restore parameters at build-once

test=develop

* This PR adds broadcast for multi-process. And it could be used in dynamic graph to broadcast parameters.

* test=develop, fix con2d with no bias

* test=develop, fix conv_2d with no bias will have null shape

* add var grad hook test=develop

* test=develop, add gradient sort backward strategy

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

* fix mem leak when converting Tensor to numpy array
test=develop

* remove unused unittest,test=develop

* follow comments, test=develop

* fix dygraph bug,test=develop

implement dygraph.parallel.DataParallel to hook reduce op.

test=develop

test=develop

test=develop

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add NCCLParallelContext for parallel dygraph

test=develop

* test=develop, add FC and test

* test=develop, refine code