Bump Version to 0.21.0 (open-mmlab#1365)

* master

* master 0721

* add README

* 1231 bump_version

* 1231 bump_version

Bump Version to 0.20.0 (open-mmlab#1247)

Bump Version to 0.19.0 (open-mmlab#1211)

bump version to 0.18.0 (open-mmlab#1129)

Bump Version to 0.17.0 (open-mmlab#1066)

* update changelog

* Bump Version

update changelog (open-mmlab#987)

Bump version to v0.15.0 (open-mmlab#895)

Upper mmcv_minimum_version

Bump version to v0.13.0 (open-mmlab#777)

* Bump version to v0.13.0

* polish changelog

* Update changelog.md

* Update changelog.md

Co-authored-by: lizz <[email protected]>

Bump version to v0.12.0 (open-mmlab#660)