Adding variant as a keyword to config.save_pretrained

[sachinruk]

What does this PR do?

When saving a model of type PreTrainedModel via save_pretrained you can add a variant argument so that the final model name can be model.vision.safetensors or model.text.safetensors. However, this capability is not extended to configuration files. This is a PR to add this capability

Fixes # (issue) If in a file you do the following, you only get one configuration file with the last config

vision_model.save_pretrained(save_directory, variant="vision")
text_model.save_pretrained(save_directory, variant="text")

Note: Tests are failing. Will fix those issues if this PR is useful in the first place. Please let me know if this PR will be accepted if I fix the rest of the tests.

Before submitting

• [ ] 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.
• [x] Did you write any new necessary tests?

cc @fxmarty

[LysandreJik]

Hey @sachinruk, can you walk me through what use case that would enable?

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