[nnx] fix MultiMetric typing

flax/nnx/training/metrics.py

@@ -373,7 +373,7 @@ def update(self, **updates) -> None:
     for metric_name in self._metric_names:
       getattr(self, metric_name).update(**updates)
 
-  def compute(self) -> dict[str, Metric]:
+  def compute(self) -> dict[str, tp.Any]:
     """Compute and return the value of all underlying ``Metric``'s. This method
     will return a dictionary, mapping strings (defined by the key-word arguments
     ``**metrics`` passed to the constructor) to the corresponding metric value.