Fix `TRAIN_ON_PRED_BOXES` setting
Fix `TRAIN_ON_PRED_BOXES` setting

Summary: Detaching predicted boxes from gradient computation since D2 assumes that `proposal_boxes.tensor` has `require_grad=False`

Reviewed By: ppwwyyxx

Differential Revision: D19762932

fbshipit-source-id: c633345435ae4d3b28e59d5f7e4e1bfed3a9b4b5

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2