[Bugfix] Fixing max token error message for openai compatible server

[jgordley]

This PR addresses a bug where the error message is inaccurate when the user makes a ChatCompletionRequest with no max_tokens sent.

Example behavior:

from openai import OpenAI
client = OpenAI(
    base_url="http://localhost:8000/v1",
    api_key="token-abc123",
)

message_over_context_length = "Hello " * 32800 + "How are you."

completion = client.chat.completions.create(
  model="mistralai/Mistral-7B-Instruct-v0.2",
  messages=[
    {"role": "user", "content": message_over_context_length}
  ]
)

Previous response (indicates a negative value for max_tokens when it was not sent):

openai.BadRequestError: Error code: 400 - {'object': 'error', 'message': 'max_tokens must be at least 1, got -45.', 'type': 'BadRequestError', 'param': None, 'code': 400}

This is because with a negative value set for max_tokens the check immediately after for token_num + request.max_tokens misses that the context length is exceeded.

New response:

openai.BadRequestError: Error code: 400 - {'object': 'error', 'message': "This model's maximum context length is 32768 tokens. However, you requested 32813 tokens in the messages, Please reduce the length of the messages.", 'type': 'BadRequestError', 'param': None, 'code': 400}

This doesn't change the previous behavior if max_tokens is sent and the combination of max_tokens and token_num is greater than max_model_length:

openai.BadRequestError: Error code: 400 - {'object': 'error', 'message': "This model's maximum context length is 32768 tokens. However, you requested 32870 tokens (32614 in the messages, 256 in the completion). Please reduce the length of the messages or completion.", 'type': 'BadRequestError', 'param': None, 'code': 400}

---

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[jgordley]

@simon-mo would you mind taking a look at this PR? It's my first one here so I want to ensure it follows the right PR standards. It's a useful error fix for our service. Thanks!

[pzzmyc]

Very helpful, I also encountered this situation, but I couldn't wait for the author to merge it, so I just used your file

[jgordley]

Hey @simon-mo, sorry to ping again but is there anything else I can do to get this PR approved? Thanks!

[esmeetu]

LGTM! Thanks.