MMDetection V2.2.1 Release

This is the last release that requires building CUDA ops. After v2.3, all CUDA ops will be moved to MMCV, so that becomes a pure python library. This make it easier to install and run this library.

New Contributors

• @AlphonsG made their first contribution in #3194
• @nguyenhoan1988 made their first contribution in #3077

Full Changelog: v2.2.0...v2.2.1