Summary:
Fixed bug preventing RetinaNet training with num_classes != 81 from converging.

Resolves #93
Closes https://github.com/facebookresearch/Detectron/pull/176

Reviewed By: ir413

Differential Revision: D7056134

Pulled By: rbgirshick

fbshipit-source-id: d3c62ce98a23a09191dd2720bc7189ee5490635d