Add ability for Metrics to add metrics from another Metrics object (#…

…3145)

* Add notion of metrics collections, which can have other Metrics of multiple metrics be added to it

See #3138 for context and use

* right, having different arguments for the same function aren't a thing in python...

(alas, that's what I get for mostly coding in C++ for the past few years. :P)

* fixed a bug while integrating into taskmaster2

* address comments (get rid of separate class, add func to Metrics directly)

* actually do the things the last comment

parlai/core/metrics.py

@@ -642,6 +642,16 @@ def clear(self):
     def share(self):
         return {'data': self._data}
 
+    def add_metrics(self, other: "Metrics") -> None:
+        """
+        Aggregate another Metrics objects metrics into this one.
+
+        Note that it is assumed that the keys for metrics are disjoint between Metrics
+        objects.
+        """
+        for k, v in other._data.items():
+            self.add(k, v)
+
 
 class TeacherMetrics(Metrics):
     """