[Fix] type of "counts" in COCO’s compressed RLE (#9274)

* [Fix]: fix wrong padding value in cached MixUp (#9259)

* [Fix] type of "counts" in COCO’s compressed RLE 

counts in COCO’s compressed RLE format have type 'str' or 'list'

Co-authored-by: plutoyuxie <34409171+plutoyuxie@users.noreply.github.com>

mmdet/datasets/transforms/loading.py

@@ -348,7 +348,7 @@ def _process_masks(self, results: dict) -> list:
             elif isinstance(gt_mask, dict) and \
                     not (gt_mask.get('counts') is not None and
                          gt_mask.get('size') is not None and
-                         isinstance(gt_mask['counts'], list)):
+                         isinstance(gt_mask['counts'], (list, str))):
                 # if gt_mask is a dict, it should include `counts` and `size`,
                 # so that `BitmapMasks` can uncompressed RLE
                 instance['ignore_flag'] = 1