fix set truncation attribute in `__init__` of `PreTrainedTokenizerBase`
Author: SaulLu
Pull request: yes

# What does this PR do?

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This PR fixes #15440. @LSinev I've putted you as co-author of this change.


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Would love to have your review @Narsil and @LysandreJik or @sgugger 

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