Avoid type bingo in `suggest_categorical`

[xadrianzetx]

Inspired by https://github.com/optuna/optuna/pull/4143, but uses generics instead.

[ConnorBaker]

In this formulation, I was unable to get heterogenous lists to type check.

For example, [1, "fish", 2, "fish"] is an inhabitant of Sequence[CategoricalChoiceType]. However, [1, "fish", 2, "fish"] is not an inhabitant of Sequence[T] for any T in CategoricalChoiceType.

EDIT: For what it's worth, I was also unable to get this to work with something like

@overload
def suggest_categorical(self, name: str, choices: Sequence[_T]) -> _T:
    ...

@overload
def suggest_categorical(
    self, name: str, choices: Sequence[CategoricalChoiceType]
) -> CategoricalChoiceType:
    ...

def suggest_categorical(
    self, name: str, choices: Sequence[CategoricalChoiceType]
) -> CategoricalChoiceType:
    """Suggest a value for the categorical parameter.

because I could not convince the type checker that the first overload was more specific than the second (Overloaded function signature 2 will never be matched: signature 1's parameter type(s) are the same or broader).

[xadrianzetx]

Thanks for investigating this! I assumed that choices were intended to be homogeneous, but indeed it seems like they are allowed to be of multiple types at once. I'll probably update the implementation based on Optuna's master.

Edit: Alternatively https://docs.python.org/3/library/typing.html#typing.TypeVarTuple (although seems like there's no support for it prior to Python 3.11).