* [CodeStyle][py2] remove unnecessary u-prefix in string literal

* `"{}".format(x)` -> `x`

* remove duplicated dtype literals

* revert changes in data_feeder.py

* remove u-prefix in data_feeder

* revert remove duplicated dtype literals in data_feeder

* remove unnecessary convert to str

* for test

* add some comments

* refine comment

* restore a removed str conversion

* re-trigger all ci, empty commit

* [CodeStyle][py2][U004] unecessary explicit `object` inheritance in class definition

* fix an increment

* [CodeStyle][py2][U008] remove unnecessary args in `super()`

* remove remained args

* revert changes in test_pylayer_op

* Revert "revert changes in test_pylayer_op"

This reverts commit ff185a9ae738afac3b0264f61bde6c6b7f72e7c4.

* revert some changes in example code

* update config

* re-blacken python code

* temporarily disable date and diff_py_file

* skip a format

* [CodeStyle] remove all future import

* revert test_error.py

* restore future import in example code

* change fluid.mean to paddle.mean

* reverse some old code examples

* use yapf to format all python file

* yapf exclude two unittests file for they rely on writing and reading file, and format will break them

* disable diff_py_file because too many diff files cause command following failed

* add gradients_with_optimizer api

* modify gradients_with_optimizer

* add gradients_with_optimizer api into paddle.auto.backward_mode

* add gradients_with_optimizer test case

* add doc for gradients_with_optimizer

* add doc for gradients_with_optimizer

* API/OP (append_backward) error message enhancement test=develop

* polish check_type test=develop

* fix unittest failed test=develop

* merge develop test=develop

* API(fluid.gridents) error message enhancement test=develop

* fix unitest failed test=develop

* 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

* update doc, test=develop

* fix related unittests, test=develop

* fix str incompatible error, test=develop

* add backward unittest test=develop

* add sub-branch in test_backward.py test=develop

* refine code, add comment test=develop

* reconstruct TestBackward Class test=develop

* fix typo of comment test=develop

* fix REGISTER_OP_WITHOUT_GRADIENT
test=develop

* prun backward ops
test=develop

* add not_been_used_vars to no_grad_set
test=develop