[Misc] Fix seq_lens for graph capture

[WoosukKwon]

Missed in #19106

[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.

🚀

[gemini-code-assist]

Code Review

This pull request fixes a bug in CUDA graph capturing within _dummy_run. The previous implementation could leave stale data in the seq_lens GPU tensor, as it only copied a slice of the data. The change ensures the entire tensor is updated, which is crucial for the correctness of CUDA graph replay. This aligns the behavior with the regular execution path and improves robustness. The change is correct and well-implemented.