[Structured Outputs] Refactor bitmask construction into get_grammar_bitmask

[WoosukKwon]

This PR refactors the scheduler's bitmask handling into the new get_grammar_bitmask method. I find this cleaner because bitmask construction is orthogonal to the scheduling logic.

This will also help #23233

[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 refactors the logic for constructing the grammar bitmask into a new get_grammar_bitmask method within the Scheduler class. This is a good improvement that enhances code clarity and modularity by separating the bitmask creation from the main scheduling logic. The implementation correctly moves the existing logic without introducing any functional changes. The code is well-structured and the refactoring achieves its goal effectively. I have reviewed the changes and found no issues.

[russellb]

lgtm!

[andylolu2]

@WoosukKwon I believe there's a bug here, we should be passing itertools.chain(scheduled_new_reqs, scheduled_running_reqs, scheduled_resumed_reqs) instead of self.running to self.get_grammar_bitmask.

As indicated by this comment

vllm/vllm/v1/core/sched/scheduler.py

Lines 551 to 553 in 1fdd5c4

	# Since some requests in the RUNNING queue may not be scheduled in
	# this step, the total number of scheduled requests can be smaller than
	# len(self.running).

self.running != scheduled_new_reqs + scheduled_running_reqs + scheduled_resumed_reqs).