* refine en doc. test=develop. test=document_fix

* Refine StaticRNN api-doc-en

* refine static rnn doc

* refine api doc of fluid.data

* refine api doc from commends

* refine api.spec

* Refine grammer

Fix english doc api, invloves the op of retinanet_target_assign, sigmoid_focal_loss and retinanet_detection_output. (#20250)

* test=document_fix
test=develop
Fix english doc api, invloves the op of retinanet_target_assign, sigmoid_focal_loss and retinanet_detection_output.

* test=document_fix
test=develop
remove Notice: this OP supports CPU mode only in english doc api of retinanet_target_assign and retinanet_detection_output

* test=document_fix
test=develop
fix API Difference for retinanet_target_assign and retinanet_detection_output

* fix API.spec conflicts in english doc api of retinanet_target_assign, sigmoid_focal_loss,  retinanet_detection_output
test=develop
test=document_fix

* fix API.spec conflicts in english doc api of retinanet_target_assign, sigmoid_focal_loss, retinanet_detection_output
test=develop
test=document_fix

* refine profiler document

test=develop test=document_fix

* update profiler document

test=develop test=document_fix

* test=document_fix
Fix english doc api, invloves the op of greater_equal,greater_than,less_equal,not_equal,
rank,rsqrt,diag,linspace,reduce_all,reduce_any,sign,where,zeros_like,unique_with_counts.

* Fix some format problem in the op of sign and greather_than.
test=develop
test=document_fix

* Fix the example of zeros_like, and update api.spec
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

* test=develop, fix docker with paddle nccl problem

* test=develop, refine en_doc for Variable and Program

* test=document_fix, fix English doc for Variable and Program

* test=document_fix, refine astype code block style

* test=document_fix, add example code for Variable properties

* polish en api,test=develop,test=document_fix

* modified api.spec, test=develop,test=document_fix

* modified ops for arc,test=develop,test=document_fix

* update for 1.6, test=develop,test=document_fix

* update API.spec

* Update API.spec

test=develop,test=document_fix

* Update API.spec

test=develop,test=document_fix

* refine Normal Uniform documnet

* Polish en doc of LoDTensorArray, test=develop, test=document_fix

* follow comments, test=develop, test=document_dix

* fix fluid.data code example, test=develop, test=document_preview, test=document_fix

* improve the docs of load_inference_model&load_params&load_persistables&chain.

enhance shape error messages for reshape op.
test=develop

* polish executor doc
test=develop

* remove fc_grad, test=develop

* remove fsp op since no unittests, test=develop

* add api check in fc test=develop

* enforce shape error info of sum op test=develop

* fix spelling test=develop

* print x_dims info test=develop

* enhance shape error info test=develop

* Add detailed error messages for nce layer

test=develop

* Fix some problems

test=develop

* Fix unit test coverage

test=develop

test=develop

* fix en_doc of one-hot and embedding test=develop, test=document_fix

* modify into fluid.data test=develop, test=document_fix

* modify api.spec test=develop, test=document_fix

* fix api.spec conflict, test=develop, test=document_fix

* elu input check
test=develop

* 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

* polish append_backward, test=document_fix

* test=document_fix, test=develop

* test=document_fix, test=develop

* polish append_backward, test=document_fix, test=develop

* fix fluid.data code example, test=develop, test=document_preview, test=document_fix

* use None instead of -1 in shape, test=develop, test=document_preview, test=document_fix