make sure cityscapes's gt_dir is local
make sure cityscapes's gt_dir is local

Summary: otherwise, reading all the json file will be very slow

Reviewed By: rbgirshick

Differential Revision: D20906336

fbshipit-source-id: f21652a1532ff1bcbab5074c432317ca02354cc7

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2