[Fix] Fix bug of not save-best in iteration-based training (#341)

* fix bug of not save-best in iteration-based training

* revise the unit test

mmengine/hooks/checkpoint_hook.py

@@ -186,8 +186,13 @@ class CheckpointHook(Hook):
             self._save_checkpoint(runner)
 
     def after_val_epoch(self, runner, metrics):
-        if not self.by_epoch:
-            return
+        """Save the checkpoint and synchronize buffers after each evaluation
+        epoch.
+
+        Args:
+            runner (Runner): The runner of the training process.
+            metrics (dict): Evaluation results of all metrics
+        """
         self._save_best_checkpoint(runner, metrics)
 
     def _get_metric_score(self, metrics):

tests/test_hook/test_checkpoint_hook.py

@@ -142,6 +142,41 @@ class TestCheckpointHook:
         assert 'best_score' in runner.message_hub.runtime_info and \
             runner.message_hub.get_info('best_score') == 1.0
 
+        # test behavior when by_epoch is False
+        runner = Mock()
+        runner.work_dir = tmp_path
+        runner.iter = 9
+        runner.model = Mock()
+        runner.message_hub = MessageHub.get_instance(
+            'test_after_val_epoch_by_epoch_is_false')
+
+        # check best ckpt name and best score
+        metrics = {'acc': 0.5, 'map': 0.3}
+        eval_hook = CheckpointHook(
+            interval=2, by_epoch=False, save_best='acc', rule='greater')
+        eval_hook.before_train(runner)
+        eval_hook.after_val_epoch(runner, metrics)
+        assert eval_hook.key_indicator == 'acc'
+        assert eval_hook.rule == 'greater'
+        best_ckpt_name = 'best_acc_iter_10.pth'
+        best_ckpt_path = eval_hook.file_client.join_path(
+            eval_hook.out_dir, best_ckpt_name)
+        assert 'best_ckpt' in runner.message_hub.runtime_info and \
+            runner.message_hub.get_info('best_ckpt') == best_ckpt_path
+        assert 'best_score' in runner.message_hub.runtime_info and \
+            runner.message_hub.get_info('best_score') == 0.5
+
+        # check best score updating
+        metrics['acc'] = 0.666
+        eval_hook.after_val_epoch(runner, metrics)
+        best_ckpt_name = 'best_acc_iter_10.pth'
+        best_ckpt_path = eval_hook.file_client.join_path(
+            eval_hook.out_dir, best_ckpt_name)
+        assert 'best_ckpt' in runner.message_hub.runtime_info and \
+            runner.message_hub.get_info('best_ckpt') == best_ckpt_path
+        assert 'best_score' in runner.message_hub.runtime_info and \
+            runner.message_hub.get_info('best_score') == 0.666
+
     def test_after_train_epoch(self, tmp_path):
         runner = Mock()
         work_dir = str(tmp_path)