Remove T5 dependency from mT5 model
Author: SD-13
Pull request: yes

# What does this PR do?

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Fixes part of #19303 
This PR removes T5 dependency from the mT5 model.


## Before submitting
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## Who can review?

@sgugger 

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Models:

- text models: @ArthurZucker and @younesbelkada
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Comments:
_The documentation is not available anymore as the PR was closed or merged._

@sgugger, I have made your suggested changes, please take a look. Thanks,

Thanks! You'll just need to add `MT5Stack` in [this list](https://github.com/huggingface/transformers/blob/56397471b454e8707b7865cfba0130f04a889592/utils/check_repo.py#L37) (along with T5Stack) to make all checks pass.

Failure is flaky so merging. Thanks for the work on this!