Add comet metric
Hey! I decided to add our new Crosslingual Optimized Metric for Evaluation of Translation (COMET) to the list of the available metrics.

COMET was [presented at EMNLP20](https://www.aclweb.org/anthology/2020.emnlp-main.213/) and it is the highest performing metric, so far, on the WMT19 benchmark.

We also participated in the [WMT20 Metrics shared task ](http://www.statmt.org/wmt20/pdf/2020.wmt-1.101.pdf) where once again COMET was validated as a top-performing metric. 


I hope that this metric will help researcher's and industry workers to better validate their MT systems in the future 🤗 !

Cheers,
Ricardo




I also thought a bit about the fact that "sources" can't be added to the batch.. but changing that would require a lot more changes. And I agree that the idea of adding them as part of the references is not ideal. Conceptually they are not references.

I would keep it like this for now.. And in the future, work on a more consistent batch interface.