Fix lad repeatedly output warning message (open-mmlab#6584)

Author: hhaAndroid

mmdet/models/dense_heads/lad_head.py

@@ -55,7 +55,7 @@ def get_label_assignment(self,
         """
 
         featmap_sizes = [featmap.size()[-2:] for featmap in cls_scores]
-        assert len(featmap_sizes) == self.anchor_generator.num_levels
+        assert len(featmap_sizes) == self.prior_generator.num_levels
 
         device = cls_scores[0].device
         anchor_list, valid_flag_list = self.get_anchors(