A small improvement in `metrics_sample.py::ROUGE` (#217)
To fix a tiny bug and do a small improvement to ROUGE class.
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Co-authored-by: Clémentine Fourrier <22726840+clefourrier@users.noreply.github.com>
Co-authored-by: Nathan Habib <30601243+NathanHB@users.noreply.github.com>
