Add FrugalScore metric
@lhoestq 

The model used by default (`moussaKam/frugalscore_tiny_bert-base_bert-score`) is a tiny model.

I still want to make one modification before merging.
I would like to load the model checkpoint once. Do you think it's a good idea if I load it in `_download_and_prepare`? In this case should the model name be the `self.config_name` or another variable say `self.model_name` ?  OK, I added a commit that loads the checkpoint in `_download_and_prepare`. Please let me know if it looks good.  @lhoestq is everything OK to merge?  I triggered the CI and it's failing, can you merge the `master` branch into yours ? It should fix the issues.

Also the doctest apparently raises an error because it outputs `{'scores': [0.6307542, 0.6449357]}` instead of `{'scores': [0.631, 0.645]}` - feel free to edit the code example in the docstring to round the scores, that should fix it @lhoestq hope it's OK now
This pull request add FrugalScore metric for NLG systems evaluation.

FrugalScore is a reference-based metric for NLG models evaluation. It is based on a distillation approach that allows to learn a fixed, low cost version of any expensive NLG metric, while retaining most of its original performance.

Paper: https://arxiv.org/abs/2110.08559?context=cs
Github: https://github.com/moussaKam/FrugalScore

@lhoestq 