add clear_gradient、current_step_lr api and parameter_list parameter for optimizer in dygraph (#1694)

* add clear_gradient function and parameter_list parameter for optimizer, test=develop

* fix AdagradOptimizer_cn.rst, test=develop

* add sample code, test=develop

* polish sample code, test=develop

* polish description for clear_gradient, test=develop

* add current_step_lr api for optimizer, test=develop

* polish backward api doc test=develop, test=document_preview

* polish backward api doc test=develop, test=document_preview

* polish doc of optimizer test=develop, test=document_preview

* polish doc of optimizer test=develop, test=document_preview

* polish doc of optimizer test=develop, test=document_preview

* polish code of no_grad_set test=develop

* update adam_op, scale_op, batch_norm_op doc. test=develop

* fix batch_norm example indent. test=develop

This reverts commit a29dcbec.

* polish adam optimizer zh api doc

* polish details

* polish seven minimize api zh doc

* polish details

* enrich grad_clip param comment

* fixed one error of api_cn

* fixed some errors of api_cn

* fixed some errors of api_cn

* fixed two errors.

* fixed two errors

* fixed one error.

* fixed one error

* fixed one error.

* fixed two errors.

* fixed one error.

* fixed two errors

* fixed two links