Update FillMaskPipeline.__call__ signature and docstring

[alvarobartt]

What does this PR do?

This PR updates the FillMaskPipeline.__call__ signature to remove the unused *args and updates the docstrings accordingly, to use inputs over args.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline Pull Request section?
• 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?

Who can review?

As it's related to a pipeline I'm assuming @Rocketknight1 (?), let me know otherwise!

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Rocketknight1]

Yes, good spot! The existing docstring is quite confusing.