fix: torch.masked_fill

[Kacper-W-Kozdon]

PR Description

Changed the way type-casting was done in the frontend. Previous implementation did not enforce keeping the input's dtype and paddle backend utilises type promotion in the function called by torch's frontend .masked_fill(). This resulted in the failure of dtype assertions and a failed paddle pytest. All of the backends use .where() to get the same results as torch.masked_fill(), with paddle's solution being probably a bit overcomplicated- but the tests for .where() are passing, so I did not want to alter that.

Potentially to-do:

• if it's not too redundant: adding .masked_fill() for the superset behaviour (even torch backend uses .where() when .masked_fill() is called);
• reformatting paddle's backend implementation of .where().

Related Issue

Closes #28437

Checklist

• Did you add a function?
• Did you add the tests?
• Did you run your tests and are your tests passing?
• Did pre-commit not fail on any check?
• Did you follow the steps we provided?

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[Ishticode]

lgtm. I will run a further check on this later today. Thanks for the effort and patience @Kacper-W-Kozdon :)

[Ishticode]

Thanks @Kacper-W-Kozdon :)