Add `CharacTER`

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## 🚀 Feature

Add `CharacTER`, a text metric used for NMT evaluation.

**Sources:**
Paper - [CharacTER: Translation Edit Rate on Character Level](https://aclanthology.org/W16-2342.pdf)
[Repo](https://github.com/rwth-i6/CharacTER)

### Motivation / Abstract

Recently, the capability of character-level evaluation measures for machine translation output has been confirmed by several metrics. This work proposes translation edit rate on character level (CharacTER), which calculates the character level edit distance while performing the shift edit on word level. The novel metric shows high system-level correlation with human rankings, especially for morphologically rich languages. It outperforms the strong CHRF by up to 7% correlation on different metric tasks. In addition, we apply the hypothesis sentence length for normalizing the edit distance in CharacTER, which also provides significant improvements compared to using the reference sentence length. ([Wang et al., 2016](https://aclanthology.org/W16-2342.pdf))

Start with the linked CharacTER paper and reference repository to determine the metric's algorithm and expected behavior. Done means torchmetrics provides the requested CharacTER NMT evaluation metric, including character-level edit distance, word-level shifts, and hypothesis-length normalization.

Written by the indexing model from the issue text.