[DLMED] enhance metrics in workflow (#3341)

Signed-off-by: Nic Ma <nma@nvidia.com>

monai/engines/workflow.py

@@ -216,7 +216,7 @@ def _register_metrics(self, k_metric: Dict, add_metrics: Optional[Dict] = None):
         if not isinstance(k_metric, dict):
             raise TypeError(f"key_metric must be None or a dict but is {type(k_metric).__name__}.")
         self.state.key_metric_name = list(k_metric.keys())[0]
-        metrics = k_metric
+        metrics = dict(k_metric)
         if add_metrics is not None and len(add_metrics) > 0:
             if not isinstance(add_metrics, dict):
                 raise TypeError(f"additional metrics must be None or a dict but is {type(add_metrics).__name__}.")