Implement text.generate.format constrained generation

[rlouf]

text.generate.format takes a model, a Python type and returns a generator which, given a prompt, will output text in the format described by the Python type:

from datetime import datetime

from outlines import models
from outlines import text


model = models.transformers("gpt2")
result = text.generate.format(model, datetime.date)("Which day is today?")
# 2023-11-12

Closes #346.

TODO

• int
• float
• datetime.datetime
• datetime.date
• datetime.time
• bool
• Add some documentation + README