Unify retinanet results format with the one used in test_engine
Unify retinanet results format with the one used in test_engine

Reviewed By: rbgirshick

Differential Revision: D6828915

fbshipit-source-id: dfc8bd970c0555d56bfd7ad313550b0a12c154f2

facebookresearch/Detectron,facebookresearch/Detectron,facebookresearch/Detectron