* [Refactor] Refactor the import rule

* minor refinement

* add a comment

[Refactor]: migrate LMDB backends and modifications from open-mmlab/mmcv#1764 and  open-mmlab/mmcv#1747 (#426)

* Add fileio ut

* pick modification from master branch

* pick modification from mmcv #1764

* test fileio without mmcv

* remove unnecessary checking

* Add pyupgrade pre-commit hook

* fix ut

* remove comments

* move fileio directory from utils

* remove CephBackend from fileio and clear some deprecated docstring

* fix format

* fix a spellling error