Fix: Echo without asking for new tokens or logprobs in OpenAI Completions API

[matheper]

Hi,

I've noticed that a recent modification to the OpenAI entrypoint has made these two use cases inoperable, which were previously supported by vLLM:

1. Echoing the prompt without generating anything (max_tokens=0).
2. Echoing along with generation, but without logprobs.

This PR adds these use cases back by adding the prompt to the engine.generate, resolving an issue of referencing logprobs when they are non-existent, and fixes the usage calculation. Closes #2706.

Here are the error logs of both use cases prior to this PR:

1. echo=True, max_tokens=0

ERROR:    Exception in ASGI application
Traceback (most recent call last):
  ...
  File "/vllm/entrypoints/openai/serving_completion.py", line 341, in create_completion
    response = request_output_to_completion_response(
  File "/vllm/entrypoints/openai/serving_completion.py", line 193, in request_output_to_completion_response
    choice_data = CompletionResponseChoice(
  File "/.env/lib/python3.10/site-packages/pydantic/main.py", line 171, in __init__
    self.__pydantic_validator__.validate_python(data, self_instance=self)
pydantic_core._pydantic_core.ValidationError: 1 validation error for CompletionResponseChoice
text
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.6/v/string_type

2. echo=True, max_tokens>0, logprobs=None

File "/vllm/entrypoints/openai/serving_completion.py", line 177, in request_output_to_completion_response
    top_logprobs = prompt_logprobs + output.logprobs
TypeError: unsupported operand type(s) for +: 'NoneType' and 'NoneType'

[matheper]

Hi @simon-mo , could you take a look at this PR when you have a chance? It fixes #2706. Any insights would be appreciated. Thanks!

[DarkLight1337]

Sorry for the long wait! From a quick search, it appears that since then, a related PR (#3441) has been merged, but it does not fully address the cases mentioned in this PR.

If you're still interested, update your code and I'll help get your changes merged.

[github-actions]

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. @matheper please rebase it. https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork