[ExecuTorch][WebGPU] SDPA test suite: replay + dynamic input_pos + in-graph KV cache

[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 WebGPU SDPA test coverage as its own diff, stacked on the SDPA op (which already carries the dynamic-input_pos consumption) and the SymInt mechanism below it: multi-step prefill->mt->decode replay, runtime-dynamic input_pos (autoregressive decode), and an in-graph mutable KV cache, each compared against a torch F.scaled_dot_product_attention golden.

• test/ops/sdpa/test_sdpa.py: ReplaySeq/REPLAY_SEQS + per-step replay export/golden; DynamicSdpaModule + export_dynamic_decode (one .pte, input_pos supplied at runtime as a SymInt); DecodeCacheModule + export_incache_decode (KV cache as register_buffer mutable buffers, so the cache persists in-graph and forward() feeds only the new token + input_pos).
• test/test_webgpu_native.cpp: test_sdpa_replay, test_sdpa_dynamic_decode (+ negative control: a pinned input_pos diverges), test_sdpa_incache_decode (+ static control: a fresh Module per step diverges, proving in-graph accumulation is real), test_symint_roundtrip, test_resize_hook; shared per-element tolerance sdpa_within_tol (abs 1e-4 OR rel 1e-3).
• test/test_build_webgpu.sh: export the replay / dynamic / in-graph-cache models for the native test.
  @exported-using-ghexport

Differential Revision: D107595144

Differential Revision: D107595144

[pytorch-bot]

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❌ 1 Unclassified Failure

As of commit 9656d0e with merge base ff2bf9c ():

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'

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