[Bugfix] add force_attention comment

[dragondream-chen]

What this PR does / why we need it?

Add the value "in_graph_capturing " comment

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.12.0
• vLLM main: vllm-project/vllm@ad32e3e

[github-actions]

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[gemini-code-assist]

Code Review

This pull request introduces a bugfix for speculative decoding with graph capture. The changes correctly propagate the graph capture status to the MTP proposer and adjust the logic within the proposer to only capture the first speculative token generation, which is the intended behavior. A helpful comment has also been added to clarify the purpose of the force_attention flag. The fix appears correct and addresses the issue effectively.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.