docs: fix return type annotation of `get_default_model_revision`
Author: MarcoGorelli
Pull request: yes

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Fixes #35981 


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Comments:
Yes, this looks good! Rebased the PR and enabled doctests

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_35982). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Hi @MarcoGorelli, the rebase fixed most issues but the failing TF test looks related - maybe the assumption that the return value is always Tuple[str, str] is not true in some TF cases?

thanks @Rocketknight1 for your review!

the traceback shows
```
>       with open(vocab_file, encoding="utf-8") as vocab_handle:
E       TypeError: expected str, bytes or os.PathLike object, not NoneType
```
and I wouldn't have expected the type annotation to influence that. Taking a look anyway

Any chance you could help me with running the test please? I tried running
```
RUN_PIPELINE_TESTS=1 pytest tests/models/whisper/test_modeling_tf_whisper.py::TFWhisperModelTest::test_pipeline_feature_extraction
```
but it still gets skipped

Hi @MarcoGorelli, tests are passing now! Were the failures earlier just a coincidence that were solved with a rebase?

Awesome! I think so

LGTM, in that case! cc @arthurzucker @Cyrilvallez for core maintainer review