Add bleurt
Sorry one nit: Could we use named arguments for the call to BLEURT?

i.e. 
        scores = self.scorer.score(references=references, candidates=predictions)

(i.e. so it is less bug prone)
 Following up on Ankur's comment---we are going to drop support for
positional (not named) arguments in the future releases because it seems to
cause bugs and confusion. I hope it doesn't create too much of a mess.

Le jeu. 30 juil. 2020 à 10:44, ankparikh <notifications@github.com> a
écrit :

> Sorry one nit: Could we use named arguments for the call to BLEURT?
>
> i.e.
> scores = self.scorer.score(references=references, candidates=predictions)
>
> (i.e. so it is less bug prone)
>
> —
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>
 > Following up on Ankur's comment---we are going to drop support for positional (not named) arguments in the future releases because it seems to cause bugs and confusion. I hope it doesn't create too much of a mess. Le jeu. 30 juil. 2020 à 10:44, ankparikh <notifications@github.com> a écrit :
> […](#)
> Sorry one nit: Could we use named arguments for the call to BLEURT? i.e. scores = self.scorer.score(references=references, candidates=predictions) (i.e. so it is less bug prone) — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <[#455 (comment)](https://github.com/huggingface/nlp/pull/455#issuecomment-666414514)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ABTMRNGAN2PMECS5K4DIHJDR6GBMLANCNFSM4PL323FA> .

Changed @ankparikh @tsellam, thanks for taking a look! We should avoid positional arguments in metrics on our side as well. It's a dangerous source of errors indeed.
This PR adds the BLEURT metric to the library.

The BLEURT `Metric` downloads a TF checkpoint corresponding to its `config_name` at creation (in the `_info` function). Default is set to `bleurt-base-128`.

Note that the default in the original package is `bleurt-tiny-128`, but they throw a warning and recommend using `bleurt-base-128` instead. I think it's safer to have our users have a functioning metric when they call the default behavior, we'll address discrepancies in the issues/discussions if it comes up.

In addition to the BLEURT file, `load.py` was changed so we can ask users to pip install the required packages from git when they have a `setup.py` but are not on PyPL

cc @ankparikh @tsellam