Add ability for Metrics to add metrics from another Metrics object

parlai/core/metrics.py

@@ -643,7 +643,17 @@ def share(self):
         return {'data': self._data}
 
 
-class TeacherMetrics(Metrics):
+class MetricsCollection(Metrics):
+    """
+    Helper container for Metrics that can have other metrics added to it.
+    """
+
+    def add_metrics(self, other: Metrics) -> None:
+        for k, v in other.report().items():
+            self.add(k, v)
+
+
+class TeacherMetrics(MetricsCollection):
     """
     Helper container which encapsulates standard metrics (F1, BLEU, ...).
     """