## Motivation
Support SegNeXt.
Due to many commits & changed files caused by WIP too long (perhaps it
could be resolved by `git merge` or `git rebase`).
This PR is created only for backup of old PR
https://github.com/open-mmlab/mmsegmentation/pull/2247
Co-authored-by: MeowZheng <meowzheng@outlook.com>
Co-authored-by: Miao Zheng <76149310+MeowZheng@users.noreply.github.com>