[MRV2][Spec Decode] DFlash

[benchislett]

Purpose

Implement DFlash for ModelRunnerV2, with full cudagraph support.

Performance result:
MRV1:

MRV2:

Running Qwen3 8B FP8 + DFlash on 1xGB200. ~1.2x speedup, from 5.2ms to 4.3ms per step (on independent benchmark, separate from profiling run).

Testing

Ran existing DFlash correctness testing with MRV1 and MRV2, got pass and identical acceptance rates.

Enabled E2E DFlash correctness and AR regression tests for both MRV1 and MRV2, both passing locally

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[mergify]

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

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

[mergify]

Hi @benchislett, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[TheEpicDolphin]

This code looks good overall. I left a few suggestions to reduce code duplications, and flagged a potential perf improvement

[TheEpicDolphin]

Looks good to me, thank you! cc: @WoosukKwon