* fix grad clip， clip op belongs to Backward op when running in Parameter Server mode.

* fix GradientClipByNorm english doc
test=develop
test=document_fix

* Fix ErrorClipByValue, GradientClipByGlobalNorm, GradientClipByValue

* add API.spec

* fix en docs of apis, test=develop, test=document_fix

* follow chunwei's comments, test=develop

* test=develop, fix docker with paddle nccl problem

* test=develop, Add Variable api and refine dygraph related API

* test=develop, Add Variable api and refine dygraph related API

* test=develop, refine test for new api and error info

* test=develop, refine error info and test_layers

* test=develop, add API.spec

* test=devleop, fix to_string python2 and python3 compat error and refien doc

* test=devleop, add API spec

* test=devleop, update API spec

* test=devleop, update API spec

* test=develop, invoke ci

* test=develop, fix example code

* test=develop, update API spec

* test=develop, add compat test and fix inplace campat dict error

* expose set_gradient_clip, test=develop, test=document_preview, test=preview

* expose gradient clip, test=develop, test=document_fix

* refine doc, test=develop

* follow lanxiang's comments, test=develop, test=document_fix

* test=develop, fix docker with paddle nccl problem

* test=develop, hide no_support api and add ut for it

* fix regularizer lod bug; test=develop

* fix exception bug and one_hot expand; test=develop

* [DOC] Fix api docs, test=develop, test=document_preview

* [DOC] Fix api annotation: fluid.layers.tensor_array_to_tensor. test=develop, test=document_preview

* test=develop, test=document_preview

update MD5 of tensor_array_to_tensor

* BugFix: fix api comment of paddle.fluid.clip.GradientClipByValue
* test=develop, test=document_preview

* expose square
test=develop

* fix activation
test=develop

* Add square API
test=develop

* add necessary op

* code refine

* fix API.spec
test=develop

* fix unit test
test=develop

* add unit test sparse_grad_clip
test=develop

* fix API.spec
test=develop

* remove mac test for test_gradient_clip
test=develop

* remove selectedrows_mul_tensor
test=develop

* fix api format and examples

test=develop

* Update executor.py

test=develop

* Update nn.py

* Update nn.py

test=develop

* Update nn.py

test=develop

test=develop

* Yuyang fix global gradient clip

* Share LoDs

* Revert unnecessary changes

* Fix bug in sequence_slice_op

* hide operator API

* update

* update api.spec

* fix merge

* fix test

* hide program APIs

* fix merge error

* update

* remove block api

* remove clone_variable

* hide block inner apis

* update

* fix tests

* refine multi-thread CPU Parallel exe

* refine multi thread CPU Parallel exe

* Refine CPU version for ParallelExecutor

* add share_parameter_between_cards_

* Fix ParallelExecutor bug

* Fix unit test

* Fix parameter opt balance

* Fix with opti (param->grad)

* Add grad to op var

* Remove shard_param_between_cards

* "fix decode bug"

* "follow commnet"

* "fix error"

* "fix hook bug"

* fix based comment

* fix copyright

* fix based on comment