Wav2 vec2 phoneme ctc tokenizer optimisation (#16817)

* Solved href rendering issue in heading

Markdown references in headings such as '####' don't render well.
Replaced it with <h4>...<a></a></h> banners.

* PhonemeTokenizer optimization using phonemizer lib

The backend should only be initialized once, otherwise it is reloaded.
Added `init_backend` function, intializes a backend attribute.
Phonemize re-uses self.backend.
Should give ~10 times faster phonemization.

* formatted file with make style

* Documentation suggestion

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Update /tokenization_wav2vec2_phoneme.py based on PR suggestion

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Update CONTRIBUTING.md

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

CONTRIBUTING.md

@@ -368,8 +368,7 @@ For documentation strings, 🤗 Transformers follows the [google style](https://
 Check our [documentation writing guide](https://github.com/huggingface/transformers/tree/main/docs#writing-documentation---specification)
 for more information.
 
-#### This guide was heavily inspired by the awesome [scikit-learn guide to contributing](https://github.com/scikit-learn/scikit-learn/blob/main/CONTRIBUTING.md)
-
+**This guide was heavily inspired by the awesome [scikit-learn guide to contributing](https://github.com/scikit-learn/scikit-learn/blob/main/CONTRIBUTING.md).**
 
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