Rename best_fn to get_metric_fn in CheckpointManagerOptions.

PiperOrigin-RevId: 750499978

Author: abhishek002002

t5x/checkpoints.py

@@ -2285,7 +2285,7 @@ def best_fn(metrics):
         max_to_keep=keep,
         save_interval_steps=period,
         keep_period=force_keep_period,
-        best_fn=best_fn if metric_name_to_monitor is not None else None,
+        get_metric_fn=best_fn if metric_name_to_monitor is not None else None,
         best_mode=metric_mode,
         keep_checkpoints_without_metrics=keep_checkpoints_without_metrics,
         cleanup_tmp_directories=True,

t5x/utils_test.py

@@ -742,10 +742,10 @@ def test_create_orbax_checkpoint_manager(
         expected_keep_checkpoints_without_metrics,
     )
     if expected_metric is None:
-      self.assertIsNone(manager._manager._options.best_fn)
+      self.assertIsNone(manager._manager._options.get_metric_fn)
     else:
       self.assertEqual(
-          manager._manager._options.best_fn(metrics),
+          manager._manager._options.get_metric_fn(metrics),
           expected_metric,
       )
 
@@ -812,7 +812,7 @@ def test_create_orbax_checkpoint_manager_from_checkpointer(
     self.assertEqual(manager._manager._options.best_mode, "max")
     self.assertTrue(manager._manager._options.keep_checkpoints_without_metrics)
     self.assertEqual(
-        manager._manager._options.best_fn(
+        manager._manager._options.get_metric_fn(
             {"train/accuracy": 0.8, "train/loss": 0.1}
         ),
         0.8,