Add unified controller interface

[iojw]

Introduces a Controller interface for RouteLLM that allows it to be used as a Python SDK without having to launch a server. The controller can act as a drop-in replacement for the OpenAI client and has parity with the server API.

from routellm.controller import Controller

# Replace OpenAI client with Controller
# client = OpenAI() 
client = Controller(
  # By default, Controller uses the best performing configuration.
  routers=["mf"],
  strong_model="gpt-4-1106-preview",
  weak_model="anyscale/mistralai/Mixtral-8x7B-Instruct-v0.1",
)

client.chat.completions.create(
    # Or, you can specify the router and threshold directly as keyword arguments.
    model="router-mf-0.116"
    messages=[
        {"role": "user", "content": "Hello!"},
    ],
)

The controller also supports returning the model name to route to for a given prompt.

routed_model = controller.route(prompt="What's the square root of 144?", router="mf", threshold=0.116)

The server and evaluation framework now use Controller to manage their routers through a unified interface!

TODO:

• [ ] Update docs