Fix HuggingfaceEngine input kwargs defaults

[lxingy3]

Summary

Fixes the mutable default input_kwargs={} arguments in the HuggingFace chat engine helpers.

The affected helpers call .pop() on input_kwargs, so sharing the same default dictionary can leak state across calls that omit the argument. This changes those defaults to None and creates a fresh empty dictionary inside each method only when no kwargs are provided.

The explicit input_kwargs behavior is unchanged for callers that pass a dictionary.

Fixes #10476

Tests

• .venv\Scripts\python.exe -m ruff check src\llamafactory\chat\hf_engine.py tests\chat\test_hf_engine.py
• .venv\Scripts\python.exe -m pytest --noconftest tests\chat\test_hf_engine.py
• git diff --check

[gemini-code-assist]

Code Review

This pull request correctly replaces mutable default dictionary arguments (input_kwargs) with None in several methods of HuggingfaceEngine to avoid shared state issues, and adds an AST-based test to enforce this. The reviewer feedback is highly valuable, pointing out that to prevent mutating the caller's dictionary (especially where .pop() is used), it is safer to create a shallow copy of the dictionary using dict(input_kwargs) instead of referencing it directly.

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[gemini-code-assist]

To prevent mutating the dictionary passed by the caller (since .pop() is called on input_kwargs later in this method), it is safer to create a copy of the dictionary instead of using it directly.

Suggested change

	input_kwargs = {} if input_kwargs is None else input_kwargs
	input_kwargs = {} if input_kwargs is None else dict(input_kwargs)

[gemini-code-assist]

To prevent mutating the dictionary passed by the caller, it is safer to create a copy of the dictionary instead of using it directly.

Suggested change

	input_kwargs = {} if input_kwargs is None else input_kwargs
	input_kwargs = {} if input_kwargs is None else dict(input_kwargs)

[gemini-code-assist]

To prevent mutating the dictionary passed by the caller, it is safer to create a copy of the dictionary instead of using it directly.

Suggested change

	input_kwargs = {} if input_kwargs is None else input_kwargs
	input_kwargs = {} if input_kwargs is None else dict(input_kwargs)

[gemini-code-assist]

To prevent mutating the dictionary passed by the caller (since .pop() is called on input_kwargs later in this method), it is safer to create a copy of the dictionary instead of using it directly.

Suggested change

	input_kwargs = {} if input_kwargs is None else input_kwargs
	input_kwargs = {} if input_kwargs is None else dict(input_kwargs)

[lxingy3]

Updated the HF engine paths to copy input_kwargs before any local mutation, so caller-owned dictionaries are left unchanged. I also added an AST regression check for that pattern alongside the mutable-default check.