Provide better typing of `VariableFactory`

[williambdean]

The VariableFactory just requires the create_variable method and dims attribute to a class. However, if the class has to_dict and is registered for deserialize function, then that gives the additional benefit of serialization and deserialization. I don't know the best way to type this or if it is even an issue

Maybe there a protocol with optional methods? or something like below should also be specified:

@runtime_checkable
class SerializableVariableFactory(Protocol):
    dims: tuple[str, ...]

    def create_variable(self, name: str) -> Variable:
        ...

    def to_dict(self) -> dict[str, Any]:
        ...

[lucianopaz]

The typing of this kind of methods is like this:

Self = TypeVar("Self", bound="SerializableVariableFactory")

class SerializableVariableFactory(Protocol):
    def to_dict(self) -> dict[str, Any]:
        return {}

    @classmethod
    def from_dict(cls: type[Self], data: dict[str, Any]) -> Self:

Starting from python 3.11, typing now includes Self, which makes this easier to do.

[williambdean]

from_dict isn't required

[williambdean]

This issue has been transferred to pymc-devs/pymc-extras