[MPNet] Add slow to fast tokenizer converter
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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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 ?

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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 albert, bert, XLM: @LysandreJik
 GPT2: @LysandreJik, @patrickvonplaten
 tokenizers: @mfuntowicz
 Trainer: @sgugger
 Benchmarks: @patrickvonplaten
 Model Cards: @julien-c
 examples/distillation: @VictorSanh
 nlp datasets: [different repo](https://github.com/huggingface/nlp)
 rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)
 Text Generation: @patrickvonplaten, @TevenLeScao
 Blenderbot, Bart, Marian, Pegasus: @patrickvonplaten
 T5: @patrickvonplaten
 Rag: @patrickvonplaten, @lhoestq
 EncoderDecoder: @patrickvonplaten
 Longformer, Reformer: @patrickvonplaten
 TransfoXL, XLNet: @TevenLeScao, @patrickvonplaten
 examples/seq2seq: @patil-suraj
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 tensorflow: @jplu
 examples/token-classification: @stefan-it
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