Add Google BLEU (aka GLEU) metric
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This PR adds the NLTK implementation of Google BLEU metric. This is also a part of an effort to resolve an unfortunate naming collision between GLEU for machine translation and GLEU for grammatical error correction.
I used [this page](https://huggingface.co/docs/datasets/add_metric.html)  for reference. Please, point me to the right direction if I missed anything.
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