Use Enums for multiple choices

[rlouf]

Presentation of the new feature

To make the model choose between different options we currently use generate.choice. Here I suggest we also use Enums to represent multiple choices:

from enum import Enum


class choices(Enum):
    a: str = "Choice A"
    b: str = "Choice B"

Where does it fit in Outlines?

This will probably make more sense once the API has been refactored.