use loss_weight=dict/float for box_head
use loss_weight=dict/float for box_head

Summary: similar to RPN

Reviewed By: rbgirshick

Differential Revision: D22367132

fbshipit-source-id: 9d3dfced6943d291ca18c3d46a126d93fc06b796

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2