Fix error with custom QE COMET

[Joao-Maria-Janeiro]

When a custom QE metric is trained, the scorer.py gives the error:
parser.error("{} requires -r/--references or -d/--sacrebleu_dataset.".format(cfg.model))

This happens because it is currently comparing the given model name with _REFLESS_MODELS = ["comet-qe"] only, not allowing to use custom QE metrics.

[ricardorei]

alright, I was thinking about this and I believe that the best option is to add a method to all COMET models that check if the model is reference-free. Then instead of checking the model class, you could call that method to check if the model requires references.

[ricardorei]

That should be a abstract method in the base class and then implemented on all child classes:

• RegressionMetric
• ReferencelessRegression
• RankingMetric

[Joao-Maria-Janeiro]

Makes sense!! Submitted!