fix BertTokenizerFast `tokenize_chinese_chars` arg
Author: SaulLu
Pull request: yes

# What does this PR do?

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Fixes #15156 

This PR proposes a change that allows to adapt a bert fast tokenizer according to the `tokenize_chinese_chars` value given during the initialization (this requires to fix the `backend_tokenizer`).

This change affects the following tokenizers: 

- `BertTokenizerFast`
- `DistilBertTokenizerFast`
- `DPRContextEncoderTokenizerFast`
- `DPRQuestionEncoderTokenizerFast`
- `DPRReaderTokenizerFast`
- `SqueezeBertTokenizerFast`

## 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?

Anyone in the community is free to review the PR once the tests have passed. I would love to have your review @LysandreJik or @sgugger

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Comments:
Very nice :)