[Bugfix] Fix vLLM UsageInfo and logprobs None AssertionError with empty token_ids

[CatherineSue]

When there is no new tokens generated, Sequence.get_output_token_ids_to_return should return empty list.

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FIX #8988 (link existing issues this PR will resolve)

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[njhill]

Thanks @CatherineSue and thanks for adding the test!

[njhill]

@CatherineSue looks like the test is failing, the max context for the model used is 8k token, would that be sufficient to expose the issue?

[CatherineSue]

Oh I saw in the server fixture in test_chat.py we set --max-model-len to 8192.
I tested it locally, --max-model-len to 8192 and zephry model is not gonna trigger the issue.

In our test, we used llama3.2-1b-instruct and llama3.2-3b-instruct, with --max-model-len=131072, and a prompt with 2000 tokens can expose it. Can we use llama3.2 model in CI?

[njhill]

@CatherineSue I don't see why not, we already use other llama models elsewhere in the CI (would probably make sense to move most/all that usage to the 1B model now anyhow).

[CatherineSue]

@njhill sorry for the late updates. I was sick for the past two weeks and didn't get time to fix the ut.
I tried to use llama3.2-1b-instruct, but the CI raises:

FAILED entrypoints/openai/test_chat.py::test_chat_completion_stream_options_and_logprobs_with_long_prompts[meta-llama/Llama-3.2-1B-Instruct] - openai.NotFoundError: Error code: 404 - {'object': 'error', 'message': 'The model `meta-llama/Llama-3.2-1B-Instruct` does not exist.', 'type': 'NotFoundError', 'param': None, 'code': 404}

Emm do we need to pass specific HF_TOKEN for the CI?

[njhill]

@CatherineSue no problem, sorry to hear that and I hope you're feeling better. Actually I had just started to look at this myself, and have written a separate test that uses a smaller max_num_batched_tokens to force prompt chunking without needing a very long context. This also reproduces the issue.

In the process of doing that I also found some related bugs. In any case I'm happy to push those updates to your branch so feel free to leave this and I'll do it soon (hopefully tonight).

[njhill]

OK @CatherineSue I have pushed the additional test, as well as changes to omit redundant empty chunks being returned to the user in this case. Hopefully this is good to go now!