[ExecuTorch][WebGPU] GPU timestamp query profiling for SDPA

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][WebGPU] GPU timestamp query profiling for SDPA #20167
• [ExecuTorch][WebGPU] SDPA test suite: replay + dynamic input_pos + in-graph KV cache #20087
• [ExecuTorch][WebGPU] Add fused SDPA (sdpa_with_kv_cache) with dynamic input_pos #20086
• [ExecuTorch][WebGPU] GPU timestamp query profiling (general implementation) #20201

SDPA-specific instrumentation layered on the general GPU-timestamp infrastructure (companion diff below): tag each fused SDPA dispatch with its kernel_name so the WebGPUQueryPool can attribute on-GPU time to the attention stage that produced it. sdpa_with_kv_cache runs four chained dispatches — update_cache -> QK (attn_weights) -> softmax -> AV (compute_out); WebGPUGraph::execute() brackets each compute pass with a timestamp when the pool is active, and this diff labels each dispatch so the per-pass durations map back to the right stage. Opt-in via the WEBGPU_TIMESTAMP_QUERY env var; off by default, so the production execute() path is byte-identical. This is the per-kernel hook a forthcoming SDPA kernel benchmark will read; the benchmark itself (and any comparative numbers) is a separate follow-up.

Co-authored with Claude.
@exported-using-ghexport

Differential Revision: D107678235

Differential Revision: D107678235

[pytorch-bot]

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❌ 1 New Failure, 2 Unrelated Failures

As of commit ebf063e with merge base 5526971 ():

NEW FAILURE - The following job has failed:

• pull / unittest-nxp-neutron / linux-job (gh)
  RuntimeError: Command docker exec -t 667adb4a75b6c7cd4dd0a01a121b3a6c40992dac715c8b2a6cbb3c7daeeb81e2 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / android / build-android (gh) (trunk failure)
  Process completed with exit code 1.

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