* add clip_grad_value_ api

* add test for ClipGradByValue

* typo fix

* refine and modify clip_grad_norm_

* no_grad

* clip_

* remove g=p.grad

* bug: AssertionError: When Variable is used as the condition of if/while , Variable can only contain one element.

* add clip_grad_norm_ api.

* fix docs and some details according to the comments.

* fix code style.

* fix no_grad problem, and fix doc.

* fix code style.

* fix doc and remove type information