[ExecuTorch][WebGPU] 2D compute dispatch tests — prefill golden + fold unit test

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][WebGPU] 2D compute dispatch tests — prefill golden + fold unit test #20584
• [ExecuTorch][WebGPU] 2D compute dispatch — lift the 65535 per-dim cap (prefill path) #20583
• [ExecuTorch][WebGPU] Dynamic-shape integration test (allocate-at-max + per-op resize) #20582
• [ExecuTorch][WebGPU] Dynamic resize hook for slice_copy (dynamic start/end) #20581
• [ExecuTorch][WebGPU] Dynamic resize hook for SDPA (live seq-len S) #20580
• [ExecuTorch][WebGPU] Dynamic resize hook for view_copy #20579
• [ExecuTorch][WebGPU] Dynamic resize hooks for sigmoid and select_copy #20578
• [ExecuTorch][WebGPU] Dynamic resize hooks for add and mul #20577
• [ExecuTorch][WebGPU] Dynamic resize hook for linear_q4gsw #20576
• [ExecuTorch][WebGPU] Dynamic resize hooks for rms_norm, embedding, rope #20575
• [ExecuTorch][WebGPU] Dynamic tensor-shape resize engine core #20574
• [ExecuTorch][WebGPU] SymInt arithmetic ops (add/sub/mul/floordiv) for dynamic shapes #20573

Test coverage for the 2D dispatch fold, stacked above the cap-lift op.

Problem: The 2D fold is load-bearing index math — a wrong {x, y} means out-of-bounds writes or dropped threads — and the prefill shapes that exercise it previously threw at the 1D cap, so they were untested.

Solution: A device-free unit test for the fold arithmetic, plus two single-shot prefill SDPA golden configs that fold each kernel family.

• Before: no coverage for >65535-workgroup dispatch; llama1b_prefill_512/_2048 shapes threw at the cap
• After: fold_workgroup_count_2d unit-tested at the cap boundaries, and the two prefill shapes run as goldens

Implementation:

• test/native/test_dispatch_2d.cpp — device-free unit test for utils::fold_workgroup_count_2d: the 1D fast path, the 2D fold, the real Llama-1B QK counts at S=512 ({65535, 3}) and S=2048 ({65535, 33}), and the needs-3rd-dimension throw; asserts each {x, y} covers [0, count)
• llama1b_prefill_512 + llama1b_prefill_2048 configs appended to the byte-mirrored CONFIGS (test_sdpa.py) and kSdpaConfigs (test_webgpu_native.cpp)
• Registers webgpu_dispatch_2d_test in CMake + the native CI script

Constraints:

• The Python/C++ config entries byte-mirror each other (kept in sync)
• add shares the element-form path with QK, so it is covered structurally; a dedicated >16M-element add fold case is omitted as disproportionate

Co-authored-with: Claude Code.

Differential Revision: D109517683

[pytorch-bot]

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❌ 1 New Failure

As of commit 1f4f899 with merge base 55a71e6 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 1

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