use PathManager and PIL to load sem_seg_file
use PathManager and PIL to load sem_seg_file

Reviewed By: ppwwyyxx

Differential Revision: D21090513

fbshipit-source-id: 41ddc682deaa5966372fbe0746a13310e5dc1e53

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2