Dispatch prompt template variations on the model name

[rlouf]

When using large language models for a given task, the output structure is fixed while the details of the prompt are often model-dependent. This is especially true for chat formats and tool use. Switching models in Outlines thus requires two changes:

1. The model name in outlines.models.provider
2. The name of the prompt template

We can mitigate the potential errors (it's easy to forget to change the name of the function!) by allowing users to register prompt variations based on model names. This feature works similarly to functools.singledispatch except dispatch happens on the model name and not the type:

import prompts


@prompts.template
def simple_prompt(query: str):
    """{{ query }}"""


@simple_prompt.register("microsoft/Phi-3-medium-128k-instruct")
def simple_prompt_name(query: str):
    """Phi-3: {{ query }}"""

So that

prompt = simple_prompt("question")
print(prompt)
# question

print(simple_prompt["gpt2"]("question"))
# question

print(simple_prompt["microsoft/Phi-3-medium-128k-instruct"]("question"))
# Phi-3: question

And when calling outlines.generate.text(model)(prompt) the prompt that is specific to model is used.