[Bugfix][Frontend] Fix validation of logprobs in ChatCompletionRequest

[schoennenbeck]

This fixes the validation of logprobs and top_logprobs for ChatCompletionRequest so that the class accepts its own default values.

FIX #14351

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[DarkLight1337]

Sorry I'm quite busy now, @hmellor can you help review the PR?

[hmellor]

@schoennenbeck I agree that the class should be able to be created with its default values. What is the behaviour when log_probs=True and top_logprobs=0?

[schoennenbeck]

You will get logprobs informations for the token that was selected but not any additional logprobs info (the top_logprobs list will be empty). That is a valid combination.

[hmellor]

LGTM