[Docs] Add hooks tutorial (#38)

* [Docs] Add hooks tutorial

* improve the description about hooks

* Add OptimizerHook usages

* fix comments

* fix comments

* fix comments

* fix comments

* fix typo

* refine description

* refactor

* refactor

* add a sentence to improve docs

* briefly introduce the hook of pytorch

* refactor the order of sections

* improve the description about hooks

* improve

* fix comments

* update priorities

* improve description

* refactor title

* fix typo

* reorder priority of hook

* fix typo

* fix comments

* remove the distributed argument from SyncBuffersHook