Add MetricTracker

[mpariente]

Just because it's less code duplicate and it seems useful.
Inspired by the WerTracker in #329.

[mpariente]

@JorisCos @jonashaag some feedback?

[jonashaag]

VERY quick code review, looks ok, maybe remove the dumping (CSV/JSON) code entirely and let the user to it? It's only an additional .to_csv() call to be done by the user

[jonashaag]

At some point I will try to think of an integration of this into System for live metrics. Right now I have something similar in my recipes

[mpariente]

At some point I will try to think of an integration of this into System for live metrics. Right now I have something similar in my recipes

Yes, using Lightning for evaluation would really make things simple, and help us get rid on bash..

[mpariente]

Thanks for the review.

[mpariente]

Resolved conflicts and clean things up.

We should converge on a format for the output (dataframe, aggregate final results in dictionnary, dump to JSON, autogenerate markdown table).
Let's merge this PR and #414 and work on a common interface to both. Should we integrate WER in the metrics?