Add comet prefix to result dict's key

When using `evaluate.combine['bleu', 'comet']`, sometimes it's hard to know which keys belongs to which metric. Adding the prefix explicitly should help.

comet.py

@@ -142,4 +142,4 @@ class COMET(evaluate.Metric):
         data = {"src": sources, "mt": predictions, "ref": references}
         data = [dict(zip(data, t)) for t in zip(*data.values())]
         scores, mean_score = self.scorer.predict(data, gpus=gpus, progress_bar=progress_bar)
-        return {"mean_score": mean_score, "scores": scores}
+        return {"comet_mean_score": mean_score, "comet_segment_scores": scores}