Avoid mutating user-provided arguments in preprocessing utils
Author: LeonardoEmili
Pull request: yes

# What does this PR do?

As per title, `_merge_kwargs` removes user-provided arguments via `pop` which is fixed by copying the arguments before attempting to modify them. This happens for instance when calling the `Gemma3nProcessor` with modality-specific kwargs.

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Comments:
Hey, seems like a good idea, but two comments:

1) Please move any code below the docstring! Otherwise the docstring stops being recognized as a docstring.
2) `deepcopy()` here seems dangerous, since `kwargs` might include tensors. Since we're only worried about the dict being mutated by `pop()`, let's just do a shallow copy or call  `kwargs.copy()`

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