SSD implementation

Thank you for this awesome work! Clearly written with enthusiasm and dedication to do the best detection ecosystem.

Got two questions, while reading detectron2 sources

1) Can RetinaNet be a subclass of general single-stage meta-architecture? That way implementation of both SSD and RetinaNet can probably come naturally without duplication of code, as well as other single stage anchor-based detector.

2) Can matcher be implemented as a base class and currently-implemented Faster R-CNN matching strategy be subclassed? I'm wondering about implementing OHEM as a subclass of matcher along-side with the current matching strategy.

Once again, thank you for the work.