[Diffusion][FLUX.2] Fuse eager AdaLN and packed SwiGLU

[BBuf]

Summary

Speed up FLUX.2 eager inference with two bit-exact fusions:

• LayerNorm + modulation at the single- and double-stream AdaLN sites
• packed SwiGLU without materializing two sliced operands

Both paths retain the original expressions for torch.compile. The eager fast paths use per-signature first-use verification and fall back safely when layout, dtype or capture state is unsupported. The cleanup also makes two correctness constraints explicit:

• a previously unseen SwiGLU signature never performs verification or a host sync during CUDA graph capture
• the packed kernel rejects non-dense outer batch strides instead of flattening an unsupported view

B300 performance

FLUX.2 Klein Base, 1024x1024, seed 42, 50 steps, native backend, quality=lossless:

configuration	denoise	e2e	vs latest-main eager
latest main eager	3.7310 s	3.9472 s	baseline
#34616 only	3.2532 s	3.4687 s	+12.81% / +12.12%
#34615 + #34616	2.9738 s	3.1957 s	+20.30% / +19.04%
torch.compile with both PRs	2.9985 s	3.1777 s	+19.63% / +19.49%

With both FLUX.2 PRs, eager is 0.83% faster than compile in denoise; E2E is within 0.57% in this single-request measurement.

Measured against main dbebc1deb42b00befa3d0de67265d7003994c1ad. The B300 validation tree is identical to this PR's final Git tree.

Output comparison

The before/after images use the same prompt, seed, resolution and step count. The generated PNGs are pixel-exact and have the same SHA-256 (6a103716dd891a301409e51bc6bd6dee2d6fb997fee21bfe0785fd504d6a5a39).

latest main eager (before)	#34616 eager (after)

Validation

• B300 fusion exactness and fallback tests: 5 passed
• coverage includes production sequence lengths, strided packed input, FP16 fallback, non-dense outer strides and a new signature encountered during capture
• B300 native model run completed without backend fallback
• final changed-file pre-commit suite passed
• weight cache cleanup verified after the model runs

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CI States

Latest PR Test (Base): ❌ Run #31687601642
Latest PR Test (Extra): ✅ Run #31692846540