[Wav2vec2] Fixed tokenization mistakes while adding single-char tokens to tokenizer
Author: Muktan
Pull request: yes

# What does this PR do?

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Fixes #10622 


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## Who can review?

@patrickvonplaten 

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Comments:
Hey @Muktan,

Thanks a lot for working on this! Could you add a test that shows how your code solves the issue? :-)

It should be in `tests/test_tokenization_wav2vec2.py`

Welcome @patrickvonplaten, I will add a test that shows how the code solves the issue.