* [Refactor] Use mdformat instead of markdownlint to format markdown.
* Update unavailiable api links in tutorials
* Update CONTRIBUTING.md
* Use mdformat==0.7.9 to support Python 3.6
* Add cosine cool down lr updater
* Use ema hook
* Update decay mult
* Update configs.
* Update T2T-ViT readme and format all readme
* Update swin readme
* Update tnt readme
* Add docstring for `CosineAnnealingCooldownLrUpdaterHook`.
* Update t2t readme and metafile
* Add Res2Net from mmdet, and change it to mmcls style.
* Align structure with official repo
* Support `deep_stem` and `avg_down` option
* Add Res2Net configs
* Add metafile&README and update model zoo
* Add unit tests
* Imporve docstring.
* Improve according to comments.