move regression loss to FastRCNNOutputLayers
move regression loss to FastRCNNOutputLayers

Summary: Following work left in D20607191 (https://github.com/facebookresearch/detectron2/commit/e12b2d6534b5fb1c0e6bdc8f486986433e502d1a) to remove the need for FastRCNNOutputs

Reviewed By: theschnitz

Differential Revision: D25843595

fbshipit-source-id: 532fb85d06d8fb8d3ee61b5919221a2002f89b2c

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2