Add tokenizers class mismatch detection between `cls` and checkpoint
Author: europeanplaice
Pull request: yes

# What does this PR do?


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Fixes #12416
This PR detects a mismatch between `cls` and a checkpoint a user intends to load.
However, It can't find a mismatch when a config doesn't contain the tokenizer's information.


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Comments:
I revised the code based on your reviews. 

I want to ask you to refactor the logic.
Thank you for offering!

@SaulLu could you confirm you're happy with the changes? I think this is good to be merged on my side, thanks for the adjustments @europeanplaice.

@SaulLu @sgugger
We made a excellent job! Thank you very much for your help!