Fix DeiT Masked Image Modeling output

[alaradirik]

What does this PR do?

Fixes the output of DeiTForMaskedImageModeling and TFDeiTForMaskedImageModeling by replacing the inaccurate MaskedLMOutput with the MaskedImageCompletionOutput class.

Follow-up PR on #22152

Before submitting

• [ X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ X] Did you read the contributor guideline, Pull Request section?
• [ X] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

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[github-actions[bot]]

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