Allow to pass a function to generate.json

[rlouf]

In most cases we want to do either of two things with the JSON generated by a model:

1. Parse it, typically with a Pydantic BaseModel;
2. Call a function with the values it contains.

In the second case the user would need to define a Pydantic model just for this purpose. We could make this easier by allowing users to pass a function to generate.json and generate the JSON Schema from the functions' signature.

import outlines.text as text
import outlines.models as models 


def do_something(foo: str, bar: List[int]):
    ...

model = models.transformers("gpt2")
res = text.generate.json(model, do_something)(prompt)
do_something(**res)