Allow specifying base model as model param in OpenAI API

[tgaddair]

Closes #330.
Closes #214.

Usage:

resp = client.chat.completions.create(
    model="mistralai/Mistral-7B-Instruct-v0.1",
    messages=[
        {"role": "user", "content": "Give me a short introduction to large language model."},
    ],
    max_tokens=100,
)
print(resp.choices[0].message.content)