[Feature] Best ckpt hook (#3773)

* [Feature] Add best ckp hook

* update docstring

* Fixed evaluate

* make key_indicator default None

* add test, and OrderDict

* add test fot best checkpoint hook

* delete comments

* add more info

* fixed eval metric

* remove key_indicator

* update doc

* fixed test.py/eval_metric.py

* fixed dist

* compatible with mmcv

* more test

* revert voc0712