[MPNet] Add slow to fast tokenizer converter

[patrickvonplaten]

What does this PR do?

Fixes #9194

This PR adds a converter from slow to fast MPNetTokenizers. This way fast tokenizers can be correctly serialized and loaded again. To prevent future issues like #9194, we should maybe think about not allowing to add a "FastTokenizer" without a corresponding converter...what do you think @sgugger, @LysandreJik, @thomwolf ?

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[sgugger]

Yes, we should not allow a fast tokenizer to be merged without the converter to avoid bugs I believe. I can write a script that checks that in the CI ;-)

Edit: Though thinking more about it, there is no need for a script, just a test we can save/reload a custom tokenizer, no?