[Bug]:Question about logprobs output being 0.0 when using `vllm` sampling params

[Violettttee]

Your current environment

The output of `python collect_env.py`

[pip3] numpy==1.26.4
[pip3] nvidia-cublas-cu12==12.4.5.8
[pip3] nvidia-cuda-cupti-cu12==12.4.127
[pip3] nvidia-cuda-nvrtc-cu12==12.4.127
[pip3] nvidia-cuda-runtime-cu12==12.4.127
[pip3] nvidia-cudnn-cu12==9.1.0.70
[pip3] nvidia-cufft-cu12==11.2.1.3
[pip3] nvidia-curand-cu12==10.3.5.147
[pip3] nvidia-cusolver-cu12==11.6.1.9
[pip3] nvidia-cusparse-cu12==12.3.1.170
[pip3] nvidia-dali-cuda120==1.35.0
[pip3] nvidia-ml-py==12.560.30
[pip3] nvidia-nccl-cu12==2.21.5
[pip3] nvidia-nvjitlink-cu12==12.4.127
[pip3] nvidia-nvtx-cu12==12.4.127
[pip3] nvidia-pyindex==1.0.9
[pip3] onnx==1.15.0rc2
[pip3] optree==0.10.0
[pip3] pynvml==11.4.1
[pip3] pytorch-quantization==2.1.2
[pip3] pytorch-triton==2.2.0+e28a256d7
[pip3] pyzmq==25.1.2
[pip3] torch==2.5.1
[pip3] torch-tensorrt==2.3.0a0
[pip3] torchdata==0.7.1a0
[pip3] torchtext==0.17.0a0
[pip3] torchvision==0.20.1
[pip3] transformers==4.48.0
[pip3] triton==3.1.0
[conda] Could not collect
ROCM Version: Could not collect
Neuron SDK Version: N/A
vLLM Version: 0.6.6

🐛 Describe the bug

Title: Question about logprobs output being 0.0 when using vllm sampling params

Body:

Hi,

Thanks for the amazing project!

I have a question regarding the logprobs output when using vllm's sampling parameters.
Specifically, when I pass the logprobs parameter in the request, I noticed that in the returned response, many tokens have their logprobs equal to 0.0.

Is this expected behavior?
It seems unusual because I found that the majority of generated tokens have their logprobs as 0.0.
I would have expected negative log probabilities instead.

Would you mind helping clarify under what conditions this might happen?
Thanks a lot in advance!

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[huangjj67]

I have this issues too. My vllm version is 0.6.6.post1.

[Violettttee]

I have this issues too. My vllm version is 0.6.6.post1.

it seems fixed in v 0.8 version

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