Make OpenAI client and ChatOpenAI compatible with OpenRouter API

[davidmigloz]

https://openrouter.ai/

Current blocking issues:

• OpenRouter does not send ChatCompletionResponseChoice.index field. Which is required by the OpenAI client currently
• CreateChatCompletionResponse.id is null for some models
• Tiktoken throws an exception when the model is not an OpenAI model (e.g. with mistralai/mistral-small)

Sample response using openai/gpt-3.5-turbo:

{
  "choices": [
    {
      "message": {
        "role": "assistant",
        "content": "I am an AI language model called ChatGPT. I don't have a personal name, but you can call me ChatGPT. How can I assist you today?"
      },
      "finish_reason": "stop"
    }
  ],
  "model": "openai/gpt-3.5-turbo",
  "usage": {
    "prompt_tokens": 35,
    "completion_tokens": 35,
    "total_tokens": 70
  },
  "id": "gen-UERvZMd21eaHdzLlvu0vi8GGBTEG",
  "object": "chat.completion",
  "created": 1705138269
}

Sample response using mistralai/mistral-small:

{
  "choices": [
    {
      "message": {
        "role": "assistant",
        "content": "123456789\n\nHere's how I did it:\n\nI simply wrote out the numbers from 1 to 9 in order without any spaces or commas in between."
      },
      "finish_reason": "stop"
    }
  ],
  "model": "mistralai/mistral-small",
  "usage": {
    "prompt_tokens": 40,
    "total_tokens": 85,
    "completion_tokens": 45
  },
  "id": "gen-ZlfNeKmOlxmoGNMlF5kHi8Spzfx7",
  "object": "chat.completion",
  "created": 1705142544
}

[davidmigloz]

https://langchaindart.com/#/modules/model_io/models/chat_models/integrations/open_router