Summary:
Dilation value `cfg.MRCNN.DILATION` is only used to calculate padding for conv, but not assigned to the dilation argument of conv.
Closes https://github.com/facebookresearch/Detectron/pull/388

Reviewed By: rbgirshick

Differential Revision: D8160945

Pulled By: ir413

fbshipit-source-id: dca6a5cf7659f29a5d1d91e860c9dfff4963e7c3