[Bugfix] Fix correct error message when len(prompt) + max_tokens > max_model_len

[sducouedic]

Closes #33418

Fixes the error message, displaying wrong max_model_len

• For vllm instance: vllm serve ./llama-194m --max-model-len 2048
• Request:

  curl -X 'POST'   'http://localhost:8000/v1/completions' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{ "model": "./llama-194m", "prompt": "What is the capital of Paris?",  "max_tokens": 2045 }'
  

• Previously:

  {"error":{"message":"This model's maximum context length is 3 tokens. However, your request has 8 input tokens. Please reduce the length of the input messages. (parameter=input_tokens, value=8)","type":"BadRequestError","param":"input_tokens","code":400}}
  

• Now:

  {"error":{"message":"This model's maximum context length is 2048 tokens. However, your request has 8 input tokens plus 2045 'max_tokens'. Please reduce one or the other. (parameter=input_tokens, max_tokens, value=(8, 2045))","type":"BadRequestError","param":"input_tokens, max_tokens","code":400}}
  

cc: @yannicks1

[gemini-code-assist]

Code Review

This pull request provides a helpful bugfix to improve the error message when a user's request exceeds the model's maximum context length due to a long prompt and a large max_tokens value. The previous error message was misleading, and the new one correctly identifies the model's maximum context length and points to both the prompt length and max_tokens as the cause of the issue. The change is well-implemented and significantly improves user experience when encountering this validation error. The implementation is correct for the intended use case, and I have no concerns.

[DarkLight1337]

Will be addressed as part of #32863

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @sducouedic.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[DarkLight1337]

Closing as superseded by #32863, thanks for your efforts though!