[Fix] Fix potential bug in MultiImageMixDataset (#9764)

Author: Xiangxu-0103

mmdet/datasets/dataset_wrappers.py

@@ -69,7 +69,7 @@ def __init__(self,
 
         self._metainfo = self.dataset.metainfo
         if hasattr(self.dataset, 'flag'):
-            self.flag = dataset.flag
+            self.flag = self.dataset.flag
         self.num_samples = len(self.dataset)
         self.max_refetch = max_refetch