fix: avoid mutable input kwargs defaults

[he-yufeng]

Summary

Removes the shared mutable {} defaults from the HuggingFace chat engine helpers.

_process_args() and _get_scores() both consume input_kwargs with .pop(). With {} as the function default, calls that omitted input_kwargs shared the same dict object. This patch changes the four HuggingfaceEngine helper signatures to default to None and normalizes to a fresh dict in the two helpers that pop values.

Closes #10476.

To verify

• PYTHONPATH=src python -m pytest tests\chat\test_hf_engine.py -q
• python -m py_compile src\llamafactory\chat\hf_engine.py tests\chat\test_hf_engine.py
• python -m ruff check src\llamafactory\chat\hf_engine.py tests\chat\test_hf_engine.py
• git diff --check

[gemini-code-assist]

Code Review

This pull request resolves a common Python issue by replacing mutable default dictionary arguments ({}) with None for input_kwargs across several methods in HuggingfaceEngine, and adds a unit test to verify this behavior. The reviewer suggests a further improvement: instead of using input_kwargs = input_kwargs or {}, a shallow copy should be created (e.g., input_kwargs.copy()) to prevent mutating the caller's original dictionary when .pop() is called on it.

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[gemini-code-assist]

Mutating input_kwargs via .pop() directly modifies the dictionary passed by the caller, which can lead to unexpected side effects if the caller reuses the dictionary. It is safer to create a shallow copy of the dictionary.

Suggested change

	input_kwargs = input_kwargs or {}
	input_kwargs = input_kwargs.copy() if input_kwargs else {}

[gemini-code-assist]

Mutating input_kwargs via .pop() directly modifies the dictionary passed by the caller, which can lead to unexpected side effects if the caller reuses the dictionary. It is safer to create a shallow copy of the dictionary.

Suggested change

	input_kwargs = input_kwargs or {}
	input_kwargs = input_kwargs.copy() if input_kwargs else {}