chore: add type hints to utility functions in functionals.py

[guozhihao-224]

Summary

Adds complete type annotations to 6 simple utility functions in roll/utils/functionals.py.

Functions Updated

• tensor_to_cpu_visitor(obj: Any, path: tuple) -> bool
• tensor_to_cuda_visitor(obj: Any, path: tuple) -> bool
• delete_tensor_grad_visitor(obj: Any, path: tuple) -> bool
• traverse_obj(value: Any, visitor: Callable[[Any, tuple], bool], path: tuple = ()) -> None
• flatten_sum(values: list | tuple) -> float
• masked_var(values: torch.Tensor, mask: torch.Tensor, unbiased: bool = True) -> torch.Tensor

Other Changes

• Translated Chinese docstrings to English
• Added Google-style docstrings (Args, Returns, Raises)
• Added Any, Callable to typing imports

Testing

• ✅ Syntax check passed

Related Issue

Part of #387

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