Fix mypy annotations in fermion_partitioning (#1282)

[rosspeili]

First slice for #1282: correct generator return types and local variable annotations in measurements/fermion_partitioning.py. No runtime or API behavior changes.

[gemini-code-assist]

Code Review

This pull request adds type annotations to several functions in src/openfermion/measurements/fermion_partitioning.py to improve type safety and code readability. Since there are no review comments, I have no feedback to provide.

[mhucka]

Thank you for doing this!

There is one change I would recommend. Although Any is valid, it is better to define so-called generic parameter types, along the following lines:

from typing import TypeVar

T = TypeVar('T')

def pair_within(labels: list[T]) -> Generator[tuple[T, ...], None, None]:
    """Generates pairings of labels that contain each pair at least once."""
    ...

Applying this across other matching functions (like pair_between and pair_within_simultaneously) allows static analyzers to check that the output tuples contain elements of the exact same type as the input.

Would you be able to try to make the appropriate changes to this PR?