ensure items in dataset have the same order across multi machine (open-mmlab#780)

Author: uni19

mmseg/datasets/custom.py

@@ -152,6 +152,7 @@ def load_annotations(self, img_dir, img_suffix, ann_dir, seg_map_suffix,
                     seg_map = img.replace(img_suffix, seg_map_suffix)
                     img_info['ann'] = dict(seg_map=seg_map)
                 img_infos.append(img_info)
+            img_infos = sorted(img_infos, key=lambda x: x['filename'])
 
         print_log(f'Loaded {len(img_infos)} images', logger=get_root_logger())
         return img_infos