Make OpenAI models compatible with the main API

[rlouf]

In this PR I add make OpenAI models compatible with the main API, for instance:

from outlines import models, generate

model = models.openai("gpt4")
generator = generate.text(model)
answer = generator("What is 2+2?", max_tokens=2)

It is thus now straightforward to swap and OpenAI model for an Open Source model by changing only one line of code. We raise a NotImplementedError whenever it is impossible to do structured generation with the API.

Closes #374. Closes #577. Closes #582. Closes #581.

TODO

• Add tests We need Make users initialize and pass the OpenAI client to OpenAI #563
• Update the documentation