Add ChrF++

[stancld]

🚀 Feature

Add ChrF++, a text metric used for NMT evaluation.

Sources:
Paper - CHRF++: words helping character n-grams
Repo / metric is also implemented in sacrebleu

Motivation

Character n-gram F-score (CHRF) is shown to correlate very well with human relative rankings of different machine translation outputs, especially for morphologically rich target languages. (Popović, 2017)

Additional context

I've already started working on this metric, I'll open a PR soon. :]

[Borda]

I have contacted the author of m-popovic/chrF@master/chrF++.py so we may ask her if she is willing to change the license (so we could keep implementation dropped in #2668) or contribute directly to TM
or let's import https://github.com/mjpost/sacrebleu/blob/master/sacrebleu/metrics/chrf.py

[SkafteNicki]

Re closing due to PR #2701