Fix error in mixed precision training of `TFCvtModel`
# What does this PR do?

This PR fixes the issue that the `TFCvtModel` cannot be trained with `keras.fit` using `mixed-precision`. 
The issue was in this [line](https://github.com/huggingface/transformers/blob/c4bf6f38bda1de3798095515875a119298bf0611/src/transformers/models/cvt/modeling_tf_cvt.py#L96) when a random tensor is initialized without specifying the correct `dtype`.  


## 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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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.
@amyeroberts
_The documentation is not available anymore as the PR was closed or merged._