* 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 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>

* remove _optimized_guard in dygrahpe_mode test=develop

* remove comment code test=develop

* remove list append test=develop

* remove list append test=develop

* refine code, test=develop

* reuse contain_var, test=develop

* test Optimizer in dygraph, test=develop

* add optest for Optimizer in dygraph, test=develop

* fix adagrad optimizer, test=develop

* fix dpsgd optimizer, test=develop

* fix test_optimizer.py, test=develop

* fix dpsgd optimizer, this op only support cpu, test=develop

* add optest for optimizer, test=develop

* add description for dpsgd, test=develop

* add rmsprop to white_list in unused_var_check.cc, test=develop

* polish code style, test=develop

* polish code style, test=develop

* delete seed attribute for DpsgdOptimizer, test=develop

* change testing to debugging, test=develop

* add clear_gradients for Optimizer, add api description, test=develop

* fix optest for optimizer's clear_gradient interface, test=develop

* add sample code, test=develop

* polish sample code, test=develop

* update layers, test=develop

* update layers for resnet, test=develop

* fix is_test attr, test=develop

* update cycle_gan, test=develop

* update reinforcement_learning, test=develop

* update ocr, test=develop

* fix bug, test=develop

* Fix the global_step & continuous applying error in EMA

test=develop

* Fix for step 0 & add unit test, test=develop

* append optimize op in the grad block of current block if current block is in control flow. test=develop

* add conditional grad op when optimizer used in control flow. test=develop

* add comment and modify typo. test=develop

* fix append_backward to support control flow. test=develop

* add test. test=develop

* fix copy_var_to_parent_block and conditional_block_grad. test=develop

* fix bug: revert to append conditional_block_grad vars to sub grad block. test=develop

* fix bug: revert to assign var to parent block even if var already is in parent block

* fix bug: consider outputs is empty. test=develop

* move _rename_grad_ out. test=develop

* modify code according to reviews from Huihuang. test=develop

* modify code according to reviews from Jinle. test=develop

* update doc, test=develop

* fix related unittests, test=develop

* fix str incompatible error, test=develop

* update layers used in mnist dygraph model, test=develop

* fix import issue, test=develop

* add dygraph utils, test=develop

* add unittest, test=develop

* remove params in Tracer object, test=develop

* Repair failed optest, test=develop

* remove build_once & name_scope (Conv2D)
test=develop

* fix unittest
test=develop

* Conv2DTranspose

* Conv3D & Conv3DTranspose
test=develop

* Pool2D & BatchNorm

* Embedding

* LayerNorm

* GRUUnit & NCE

* PRelu

* BilinearTensorProduct

* GroupNorm & SpectralNorm

* TreeConv
test=develop

* fix LayerNorm in transformer unnittest
test=develop

* disable LayerNorm or BatchNorm in multicard
test=develop

* refine Layer.create_parameter api
test=develop

* refine LayerNorm, remove begin_norm_axis param, add normed shape check
test=develop

* LayerNorm bug fix
test=develop

* fix optest,test=develop

* fix optest, test=develop

* fix optest for pass parameter_list when constructing an Optimizer class instance, test=develop

* polish code for better code style, test=develop

* fix se_resnext optest, test=develop

* polish code for better code style, test=develop
Co-authored-by: Nsongyouwei <youwei0314@gmail.com>

* optimize adam speed by removing _finish_update test=develop

* fix SparseAdamFunctor param list test=develop

* Remove scale_op in expect_list of adam_op test=develop

* fix test optimizer loss assert error test=develop

* fix test optimizer loss assert error test=develop

* modify PADDLE_ENFORCE usage test=develop

* fix op_type in lamb_op.cc test=develop

* fix errors ostream format bug test=develop

* add betaPowOut in ngraph op test=develop

* fix ngraph::op api for gcc8 test=develop

* clean code test=develop

* modify struct into class test=develop

* remove code of beta1Tensor in lamb_op test=develop

* update layers, test=develop

* fix input numpy, test=develop

* fix bugs, test=develop

* follow commments, test=develop

* update getitem, test=develop

* add selected rows supported in framework

* Add a single_process_multi_thread transpiler.
* Add some UTs.
* Fix some API description.

* test=develop, fix docker with paddle nccl problem

* don't expose numerous Tensor.set(), test=develop

* fix condition, test=develop

* fix float16 bug, test=develop

* feed should be Tensor or np.array, not Variable or number, test=develop

* use forcecast to copy numpy slice to new array, test=develop

* remove float16-uint16 hacking, test=develop

* add variable method to varbase and refactor to_variable to support return varbase

* support kwargs in varbase constructor

* add VarBase constructor to support default python args

* refine varbase initial method

* reset branch

* fix ut for change VarBase error info to PaddleEnforce

* cherry is parameter change before

* overload isinstance to replace too many change of is_variable

* rm useless files

* rm useless code merged by git

* test=develop, fix some ut failed error

* test=develop, fix test_graph_wrapper

* add some tests, test=develop

* refine __getitem__, test=develop

* add tests, test=develop

* fix err_msg, test=develop

* support SelectedRows in dygraph, test=develop

* fix bug of _grad_ivar interface, test=develop

* add optest for support seletedrows, test=develop

* fix bug for gradient_accumulator in GPU mode, test=develop

* fix error when Selectedrows addto LodTensor in sorted_gradient mdoe in dygraph, test=develop

* refine and simplify gradient accumulator code, test=develop

* add optest, test=develop

* add optest and simplify code, test=develop

* fix bug for test_imperative_selected_rows, test=develop

* add optest for Coverage, test=develop

* fix gradient interface and simplify code, test=develop

* update api for gradient, test=develop

* fix ShareDim's bug in DygraphExecutionContext class, test=develop

* add optest, test=develop

* fix adam sample code bug test=document_fix

* fix sample code bug in scale test=document_fix

* add Adam beta1/beta2 support Variable. test=develop

* optimize the english description for APIs test=develop test=document_fix

* Fix several api docs

test=develop, test=document_fix

* add new save load interface; test=develop

* add new save interface; test=develop

* add save load interface ;

* fix save load error;

* fix dygraph set dict bug;

* add save load unit test; test=develop

* fix test_imperative_optimizer bug; test=develop

* fix unitest optimizer bug; test=develop

* fix code coverage; test=develop

* fix converage; test=develop

* add document for apis; test=develop

* fix unitest error; test=develop

* fix save load unit test error; test=develop

* fix error message; test=develop

* change set_parameter set_optimizer to save_dygraph; test=develop

* add load_graph check; test=develop

* fix api spec; test=develop

* polish minimize en doc

* polish adam optimizer en doc

* polish adamax optimizer en doc

* polish adagrad and decayed adagrad optimizer en doc

* polish model average en doc, test=develop, test=document_fix, test=document_preview

* self review and further polishing doc

* update API.spec, test=develop, test=document_fix

* update fluid.data api in examples, test=develop, test=document_fix

* update fluid.data inferface, test=develop, test=document_fix

* replace -1 by none, test=document_fix

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

* follow chunwei's comments, test=develop

Refine document of DGCMomentumOptimizer

* fix doc of apply_optimize
test=document_fix
test=document_preview

* modify doc of backward
test=develop
test=document_fix

* modify document hash
test=develop
test=document_preview

* add AdadeltaOptimizer doc, test=develop

* refine doc,test=develop

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