Fix comments for metrics_to_scalars (#8782)

metrics_to_scalars can return non-float values, such as int or complex, depending on the dtype of the tensor.

(cherry picked from commit f3442db)

Author: edward-io

pytorch_lightning/utilities/metrics.py

@@ -27,13 +27,13 @@ def metrics_to_scalars(metrics: Any) -> Any:
 
     Raises:
         MisconfigurationException:
-            If ``value`` contains multiple elements, hence preventing conversion to ``float``
+            If tensors inside ``metrics`` contains multiple elements, hence preventing conversion to a scalar.
     """
 
     def to_item(value: torch.Tensor) -> numbers.Number:
         if value.numel() != 1:
             raise MisconfigurationException(
-                f"The metric `{value}` does not contain a single element" f" thus it cannot be converted to float."
+                f"The metric `{value}` does not contain a single element, thus it cannot be converted to a scalar."
             )
         return value.item()