LastLevelP6P7 should allow custom in_feature for custom backbones (#905)
LastLevelP6P7 should allow custom in_feature for custom backbones (#905)

Summary:
LastLevelP6P7 had a hardcoded "res5" in feature key, PR adds parameter to overload this so RetinaNet FPN can be used with other backbones.
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/905

Differential Revision: D19990142

Pulled By: ppwwyyxx

fbshipit-source-id: 5dff259e13529b311036fccc7768dfa2b9f44793

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2