* add register_forward_hook api for Layer, test=develop

* fix sample code, test=develop

fix Chinese doc bug in argsort, add the dtype support of dtypes

fix sample code diff for OP(retinanet_detection_output, retinanet_target_assign), fix `gamma` default value for OP(sigmoid_focal_loss) (#1971)

* fix sample code diff for OP(retinanet_detection_output, retinanet_target_assign), fix  default value for OP(sigmoid_focal_loss). test=develop

* fix float value. test=develop

* add api_cn

* test=develop

* add paddle.tensor

* add

* fix program api sample code error

* fix detail error

* fix build strategy cn doc sample code diff

* fix code error

* fix execution strategy error

* fix LayerNorm error

fix regularizaiton documention to dispel puzzle of user

Refine the doc of create_global_var, pad, pad_constant_like and pad2d to make it consistent with en doc.

* sync recompute doc btw zh and en, test=document_preview

* test=preview

* add inplace_abn. test=develop

* fix lstm doc error

* fix lstm doc error

* remove unused line in lstm doc

* Add learning_rate in NoamDecay test=develop

* Update sample code test=develop

* add DataLoader drop_last doc, test=develop

* follow comments, test=develop, test=document_fix

* add update for gelu: approximation option

* removefill_constant_batch_size_like  gaussian_random_batch_size_like  uniform_random_batch_size_like_cn API doc test=develop

* add filter_by_instag out_val_ifempty cn api test=develop

* change out_val_ifempty to out_val_if_empty in filter_by_instag test=develop

In elementwise_min api doc has the error, all the api just use the elementwise_max op, we just correct the doc.

* add disable_dygraph enable_dygraph en cn docs
test=develop

* clean DS_Store
test=develop

* add missing get_program_persistable_vars and get_program_parameter doc
test=develop

* add missing save doc, add link
test=develop

* add tracer_option in function about profiler test=develop

Fix the pow doc bug, change the pow parameter from 2.0 to 3.0