Dflash integration

[StanislavII]

Purpose

DFlash integration in the speculative decoding pipeline related to the PRs #32206, #34014

This PR adds working DFlash support in the proposer path by handling DFlash-specific attention metadata and slot mapping correctly during the first drafting pass with supported batch size

Main changes:

• add a DFlash branch in set_inputs_first_pass()
• build DFlash query inputs as [next, mask, mask, ...]
• preserve the original packed context slot mapping from CommonAttentionMetadata

This change restores correct DFlash behavior for both single-request and batched proposer execution.

Test Result

Verified with Sglang version of Qwen3-8B-DFlash-b16 in GSM8K dataset.

Throughput / acceptance comparison

T=0 Draft Tokens = 16, Max Tokens = 2048

Batch size	Framework	tokens/s	Mean acceptance length
4	vLLM	1091	6.08
4	SGLang	1024	6.07
8	vLLM	1799	6.08
8	SGLang	1738	6.07
16	vLLM	2428	6.08
16	SGLang	2360	6.08
32	vLLM	2741	6.09
32	SGLang	2540	6.08

---

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[StanislavII]

@benchislett could you please take a look when you have a moment?

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @StanislavII.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

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[benchislett]

Hi @StanislavII, thank you for the contribution. I've had a look through the PR and it seems reasonably well implemented.

I have been tinkering with DFlash and have opened a full-support PR today, here: #36847. I intend to solicit reviews and gather opinions based on that PR.

Please consider both PRs and give a cursory review of my work, if you are available. If you feel that your implementation has certain advantages and we should move forward with this branch, please let me know as I am open to this possibility. Otherwise, please close the PR or change to a draft, and if you can leave a detailed review of my PR.

Thanks again.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @StanislavII.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork