fix import COCO_CATEGORIES in prepare_panoptic_fpn.py (#167)
fix import COCO_CATEGORIES in prepare_panoptic_fpn.py (#167)

Summary:
Simple fix, should be trivial to check.
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/167

Differential Revision: D18126144

Pulled By: ppwwyyxx

fbshipit-source-id: a23890faef0575a808d04ab2116754e97b2ed57c

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2