Assertions to exceptions
Author: MocktaiLEngineer
Pull request: yes

# What does this PR do?

This PR addresses the issue [#12789](url)
I have modified the file transformers/src/transformers/tokenization_utils.py file to throw a type error instead of assertions.

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Fixes # (issue)


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Comments:
Note that you still need to run `make style` on your branch to fix the code quality check.

> Thank you for fixing this!
> 
> Could you run `make style` and `make quality` from the root of the repo? That should fix the failing test.

Thank you so much, will do! 

> Note that you still need to run `make style` on your branch to fix the code quality check.

Yes, this is my first time contributing to a PR, so I'm sorry if I'm being a little slow. I have run the 'make style', hoping it passes this time. And thanks for approving the changes :)

No problem, we all have to learn and start somewhere :-)
The last test failure is unrelated to this PR (flaky test) so merging. Thanks again for your contribution!