* 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