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

@@ -652,6 +652,7 @@ def add_metrics(self, other: "Metrics") -> None:
         for k, v in other._data.items():
             self.add(k, v)
 
+
 class TeacherMetrics(Metrics):
     """
     Helper container which encapsulates standard metrics (F1, BLEU, ...).