Implement configurable for RetinaNet
Implement configurable for RetinaNet

Summary: Update RetinaNet init to use configurable

Reviewed By: ppwwyyxx

Differential Revision: D23824612

fbshipit-source-id: 39564903ebadadc9068d01d00db97d0db8f29001

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2