[ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Lift FastVLM dispatch limits and harden graph construction #21602
• [ExecuTorch][WebGPU] Cover Conv1d routes and dynamic manifests #21601
• [ExecuTorch][WebGPU] Support general groups=1 Conv1d #21600
• [ExecuTorch][WebGPU] Test byte-packed BOOL tails #21599
• [ExecuTorch][WebGPU] Support byte-packed BOOL storage #21598
• [ExecuTorch][Vulkan] Preserve persistent buffer mutations #21597
• [ExecuTorch][WebGPU] Preserve Linear and Q4 embedding params across dynamic resize #21483
• [ExecuTorch][WebGPU] Generate logical and arithmetic binary shader variants #21451
• [ExecuTorch][WebGPU] Generate extrema and unary shader variants #21450
• [ExecuTorch][WebGPU] Generate typed to_copy shader variants #21449
• [ExecuTorch][WebGPU] Make WGSL generation rollback-safe #21448
• [ExecuTorch][WebGPU] Add Qwen3 K16 attention and fp16 KV cache #21136
• [ExecuTorch][WebGPU] Add HuggingFace rotate-half RoPE #21135
• -> [ExecuTorch][WebGPU] Repair dynamic SDPA routing, add attestation #21134
• [ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds #21133
• [ExecuTorch][WebGPU] Add Llama K16 online causal attention #21132
• [ExecuTorch][WebGPU] Replace broad QKV fusion with BK64 kernel #21131
• [ExecuTorch][WebGPU] Route exact prefill shapes to the BK64 GEMM #21130
• [ExecuTorch][WebGPU] Fuse the SwiGLU elementwise chain #21129
• [ExecuTorch][WebGPU] Coordinate dynamic dispatch routes #21128
• [ExecuTorch][WebGPU] Add certified scoped output suppression #21127

With dynamic sequence positions, the backend could record only one SDPA route,
so a decode following a dynamic-position prefill could dispatch the wrong
kernel, and there was no way to confirm which kernel actually ran. This treats a
dynamic SymInt position (not just a dynamic sequence dimension) as sufficient to
record both SDPA routes, exposes active-kernel route attestation for correctness
and performance harnesses, and reads the timestamp-query gate per execution so
diagnostics can be enabled after module initialization. The attestation state
and query are compiled only under the WGPU_BACKEND_ENABLE_PROFILING build flag,
so production builds carry no additional state or cost, mirroring how the Vulkan
backend gates its QueryPool behind ET_EVENT_TRACER_ENABLED. No Vulkan analogue
for the routing fix (WebGPU runtime routing and observability).

Key changes:

• WebGPUGraph.{cpp,h}: expose the active-route attestation query (compile-gated
  behind WGPU_BACKEND_ENABLE_PROFILING) and read the timestamp-query gate per
  execution.
• WebGPUUtils.h, Sdpa.cpp: treat a dynamic SymInt position (not only a dynamic
  sequence dimension) as sufficient to record both SDPA routes.
  @exported-using-ghexport

Differential Revision: D113171745

Differential Revision: D113171745

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21134

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 2 New Failures, 35 Pending

As of commit 3d8e640 with merge base 28a7fac ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / test-aot / test-aot (gh)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID AAAC:330715:357FCBE:B6F4ACF:6A760B4E and timestamp 2026-08-07 16:43:58 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting
• Cadence Build & Test / cpu-test / test-ops / test-ops (gh)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID 9CE0:1600F6:37632DB:BC43A60:6A760B4F and timestamp 2026-08-07 16:43:59 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.

[SS-JIA]

Review automatically exported from Phabricator review in Meta.