[ExecuTorch][WebGPU] Add fused SDPA (sdpa_with_kv_cache) with dynamic input_pos

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [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] SymInt live-scalar mechanism + et_vk.select_as_symint #20085
• [ExecuTorch][WebGPU] Add update_cache tests (native numeric + export) #20084
• [ExecuTorch][WebGPU] Add update_cache op (llama.update_cache) #20083
• [ExecuTorch][WebGPU] Add per-pass dispatch ordering + scratch buffer tests #20080
• [ExecuTorch][WebGPU] Switch native backend from wgpu-native to Dawn (Tint) + SwiftShader #20079
• [ExecuTorch][WebGPU] Graph-owned scratch buffers for fused-op intermediates #20073
• [ExecuTorch][WebGPU] Per-pass compute dispatch ordering for fused multi-dispatch ops #20072

Adds the fused sdpa_with_kv_cache op (QK attention-weights, softmax, attention-output sub-kernels over the KV cache), composing the three enablers below it: the base graph's inter-dispatch buffer passing (scratch buffers + multi-pass execute), the update_cache op, and the SymInt live-scalar mechanism. The QK/softmax/AV kernels mirror the Vulkan reference's flat-index/GQA/causal-mask math (NCHW, buffer-only, fp32).

input_pos is consumed dynamically via the SymInt mechanism: the op reads symint_buffer() as a uniform, sizes its scratch + dispatches for the max context length, and registers a resize hook so a single delegate runs an autoregressive decode loop (feed only the new token + advancing input_pos) instead of a fixed baked position. Mirrors the Vulkan SymInt = live uniform-buffer design.

Tests live in the stacked test-suite diff above (clean op diff here).
@exported-using-ghexport

Differential Revision: D107595125

Differential Revision: D107595125

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure, 1 Unclassified Failure

As of commit 13b12d5 with merge base ff2bf9c ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t d011ce5f29398b832666dba6e0cb68c3f5bcae5faeb203c4b311e911ce87c6e9 /exec failed with exit code 139

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• Test WebGPU Native (Dawn) / test-webgpu-native / linux-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  /pytorch/executorch/backends/webgpu/test/native/test_scratch_buffer.cpp:67:3: error: use of undeclared identifier 'webgpu_wait'

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) (detected as infra flaky with no log or failing log classifier)

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