Raise exceptions instead of using assertions for control flow #12789
Author: MocktaiLEngineer
Pull request: yes

# What does this PR do?

Replaces assertions with exceptions for this file - transformers/src/transformers/tokenization_utils.py 
In accordance with this issue - https://github.com/huggingface/transformers/issues/12789

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Fixes # (issue)


## 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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