Summary:
Add TRAIN.COPY_WEIGHTS (default False):

When setting True, training will copy TRAIN.WEIGHTS to checkpoint folder and treat it as a candidate checkpoint. This is useful if we want to resume a checkpoint from another job (instead of using ImageNet pre-training).

Reviewed By: rbgirshick

Differential Revision: D8710711

fbshipit-source-id: 9c011227867c52eaaeb40e3c2679cff1cfccdc66