Update tuner.py

Resolves #642

keras_tuner/engine/tuner.py

@@ -26,6 +26,7 @@
 
 from keras_tuner import config as config_module
 from keras_tuner.engine import base_tuner
+from keras_tuner.engine import objective as obj_module
 from keras_tuner.engine import tuner_utils
 
 MAX_FAIL_STREAK = 5
@@ -275,10 +276,14 @@ def run_trial(self, trial, *args, **kwargs):
         """
         # Not using `ModelCheckpoint` to support MultiObjective.
         # It can only track one of the metrics to save the best model.
-        model_checkpoint = tuner_utils.SaveBestEpoch(
-            objective=self.oracle.objective,
-            filepath=self._get_checkpoint_fname(trial.trial_id),
-        )
+        filepath = self._get_checkpoint_fname(trial.trial_id)
+        if isinstance(self.oracle.objective, obj_module.DefaultObjective):
+            model_checkpoint = keras.callbacks.SaveModelCheckpoint(filepath)
+        else:
+            model_checkpoint = tuner_utils.SaveBestEpoch(
+                objective=self.oracle.objective,
+                filepath=filepath,
+            )
         original_callbacks = kwargs.pop("callbacks", [])
 
         # Run the training process multiple times.