[Bugfix] Fix grammar_bitmask IndexError caused by outdated apply_grammar_bitmask method

[shen-shanshan]

What this PR does / why we need it?

Fix #2033

Sync vllm-project/vllm#14702 to solve grammar_bitmask IndexError caused by outdated apply_grammar_bitmask method

Does this PR introduce any user-facing change?

No

How was this patch tested?

[gemini-code-assist]

Summary of Changes

Hello @shen-shanshan, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

I've addressed a critical bug in the apply_grammar_bitmask method within the model runner. This change resolves an IndexError by updating how grammar bitmasks are reordered and applied to logits, particularly when speculative decoding is active. The core purpose is to ensure that structured output generation remains stable and correct under various decoding scenarios, preventing crashes and improving the reliability of the system.

Highlights

• Bug Fix for Grammar Bitmasking: I've fixed an IndexError that could occur in the apply_grammar_bitmask method. This was caused by an outdated approach to reordering the grammar bitmask, especially when speculative decoding was involved. The sorted_bitmask is now correctly sized based on logits.shape[0], preventing out-of-bounds access.
• Improved Speculative Decoding Compatibility: The logic within apply_grammar_bitmask has been significantly revised to properly account for speculative decode tokens. This ensures that the grammar_bitmask is applied to all relevant tokens, including those generated speculatively, by correctly calculating logit indices with a cumulative_offset.
• Refactored Bitmask Reordering: I've refactored the process of reordering the grammar_bitmask to align with the batch order. The new approach ensures that the bitmask accurately corresponds to the current batch's request and token sequence, improving the robustness of structured output generation.

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[gemini-code-assist]

Code Review

This pull request provides a bugfix for an IndexError in the apply_grammar_bitmask method and adds support for speculative decoding. The changes appear correct and robust. The sorted_bitmask is now sized appropriately to match the logits tensor, which should resolve the reported error. The logic also correctly handles the variable number of tokens per request introduced by speculative decoding. My review includes two suggestions to improve the code's clarity and maintainability by using more descriptive variable names, which is important given the complexity of the indexing logic.

[gemini-code-assist]

For clarity and to avoid confusion, consider renaming seq to something more descriptive, like requests_in_batch_order. This is especially helpful because seq is reused later with a different meaning.

Suggested change

	seq = sorted(self.input_batch.req_id_to_index.items(),
	key=lambda x: x[1])
	for req_id, batch_index in seq:
	requests_in_batch_order = sorted(self.input_batch.req_id_to_index.items(),
	key=lambda x: x[1])
	for req_id, batch_index in requests_in_batch_order:

[gemini-code-assist]

The variable seq is reused from a previous loop. To improve readability and prevent potential bugs during future modifications, it's better to use a distinct and descriptive name, such as grammar_requests_in_mask_order.

Suggested change

	seq = sorted(scheduler_output.structured_output_request_ids.items(),
	key=lambda x: x[1])
	for req_id, _ in seq:
	grammar_requests_in_mask_order = sorted(scheduler_output.structured_output_request_ids.items(),
	key=lambda x: x[1])
	for req_id, _ in grammar_requests_in_mask_order: