Revert "remove _get_metric_scope"

This reverts commit eeb7a8c5ed.

mmengine/hooks/checkpoint_hook.py

@@ -2,6 +2,7 @@
 import os.path as osp
 import re
 import warnings
+from collections import OrderedDict
 from math import inf
 from pathlib import Path
 from typing import Callable, Dict, List, Optional, Sequence, Union
@@ -296,6 +297,18 @@ class CheckpointHook(Hook):
         """
         self._save_best_checkpoint(runner, metrics)
 
+    def _get_metric_score(self, metrics, key_indicator):
+        eval_res = OrderedDict()
+        if metrics is not None:
+            eval_res.update(metrics)
+        if len(eval_res) == 0:
+            warnings.warn(
+                'Since `eval_res` is an empty dict, the behavior to save '
+                'the best checkpoint will be skipped in this evaluation.')
+            return None
+
+        return eval_res[key_indicator]
+
     def _save_checkpoint(self, runner) -> None:
         """Save the current checkpoint and delete outdated checkpoint.
 
@@ -372,7 +385,7 @@ class CheckpointHook(Hook):
         # save best logic
         # get score from messagehub
         for key_indicator, rule in zip(self.key_indicators, self.rules):
-            key_score = metrics.get(key_indicator, None)
+            key_score = self._get_metric_score(metrics, key_indicator)
 
             if len(self.key_indicators) == 1:
                 best_score_key = 'best_score'