[tune] use default anonymous metric _metric if at least a mode is set (#12159)

Co-authored-by: Richard Liaw <rliaw@berkeley.edu>

python/ray/tune/function_runner.py

@@ -16,8 +16,8 @@ from six.moves import queue
 from ray.util.debug import log_once
 from ray.tune import TuneError, session
 from ray.tune.trainable import Trainable, TrainableUtil
-from ray.tune.result import (TIME_THIS_ITER_S, RESULT_DUPLICATE,
-                             SHOULD_CHECKPOINT)
+from ray.tune.result import (DEFAULT_METRIC, TIME_THIS_ITER_S,
+                             RESULT_DUPLICATE, SHOULD_CHECKPOINT)
 from ray.tune.utils import (detect_checkpoint_function, detect_config_single,
                             detect_reporter)
 
@@ -164,7 +164,7 @@ class StatusReporter:
             "report __call__ is made to ensure correct runtime metrics.")
 
         if _metric:
-            kwargs["_metric"] = _metric
+            kwargs[DEFAULT_METRIC] = _metric
 
         # time per iteration is recorded directly in the reporter to ensure
         # any delays in logging results aren't counted