[Bugfix] Fix benchmark script bug: inaccurate stats for vllm backend when max_model_len < input_len + output_len

[WangErXiao]

Fix benchmark script bug: inaccurate stats for vllm backend when max_model_len < input_len + output_len.

When max_model_len is smaller than prompt_len + expected_output_len, the request may terminate early without reaching the expected_output_len, resulting in inaccurate throughput statistics. This is because the total generated token count is calculated as expected_output_len multiplied by the number of prompts. So add assert max_model_len > input_len + output_len.

[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.

🚀

[ywang96]

Thanks for the fix!