None metrics (#4902)

* cat or concat

* back to cat

* Only add the metric if it is not None

* lint

parlai/core/metrics.py

@@ -292,7 +292,8 @@ def from_mask(
         cls, metric_per_token: torch.Tensor, mask: torch.Tensor
     ) -> List[Metric]:
         """
-        From token-level metrics, returns an aggregate MyMetric per example in the batch.
+        From token-level metrics, returns an aggregate MyMetric per example in the
+        batch.
 
         :param metric_per_token:
             a (batchsize x num_tokens) Tensor
@@ -1097,6 +1098,8 @@ def _consume_user_metrics(self, observation):
         # User-reported metrics
         if 'metrics' in observation:
             for uk, v in observation['metrics'].items():
+                if v is None:
+                    continue
                 if uk in ALL_METRICS:
                     # don't let the user override our metrics
                     uk = f'USER_{uk}'