Update mmdetection example to use the Core API, and to work with the latest version of mmdetection, mmcv, etc.

It looks like determined is using mmdetection v2.27 and mmcv v1.7 both of which are about a year old now.

Both libraries have released new major versions since then.

RTMDet could be a good example: https://github.com/open-mmlab/mmyolo
