Allow any annotations file name in coco extractor (#1650)

datumaro/datumaro/plugins/coco_format/extractor.py

@@ -24,7 +24,8 @@ class _CocoExtractor(SourceExtractor):
     def __init__(self, path, task, merge_instance_polygons=False):
         assert osp.isfile(path), path
 
-        subset = osp.splitext(osp.basename(path))[0].rsplit('_', maxsplit=1)[1]
+        subset = osp.splitext(osp.basename(path))[0].rsplit('_', maxsplit=1)
+        subset = subset[1] if len(subset) == 2 else None
         super().__init__(subset=subset)
 
         rootpath = ''