Conversion from slow to fast for BPE spm vocabs contained an error.
Author: Narsil
Pull request: yes

# What does this PR do?

- There is only 1 test currently (tokenizers + slow) that used the modified path
and it's reformer, which does not contain any ids modification so the
bug was silent for now.
- The real issue is that vocab variable was overloaded by
SentencePieceExtractor, leading to Slow specific vocab oddities to be
completely ignored
- The bug was reported here https://github.com/huggingface/transformers/issues/9518
- Ran the complete tokenization test suite with slow without error
(`RUN_SLOW=1 pytest -sv tests/test_tokenization_*`)
- We need to keep in mind that BPE + SPM are relatively rare.
- I still need to carry out
 a full sweep of the hub to check all possible variants.

Affected models (all repos containing `sentencepiece.bpe.model`):
- `Musixmatch/umberto-commoncrawl-cased-v1`
- `idb-ita/gilberto-uncased-from-camembert`
- `itsunoda/wolfbbsRoBERTa-large` (not fixed with current PR, seems linked to prefixed '_' in fast tokenizers)
- `itsunoda/wolfbbsRoBERTa-small` (not fixed with current PR)
- `mrm8488/umberto-wikipedia-uncased-v1-finetuned-squadv1-it`
- `EMBEDDIA/litlat-bert`
- `neuralspace-reverie/indic-transformers-bn-xlmroberta`
- `neuralspace-reverie/indic-transformers-hi-xlmroberta`


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Fixes # (issue)


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