Add COMET

[devrimcavusoglu]

Brief metric info COMET is proposed primarily as an MT evaluation metric through a trained model.

COMET is an open-source framework for MT evaluation that can be used for two purposes:

• To evaluate MT systems with our currently available high-performing metrics (check: COMET Metrics).
• To train and develop new metrics.

Meta information Please fill below if applicable:

• Repo/Custom implementation: https://github.com/Unbabel/COMET
• Python implementation available: [x]
• Paper: https://aclanthology.org/2020.emnlp-main.213/
• Huggingface: https://github.com/huggingface/datasets/blob/master/metrics/comet