[ExecuTorch][WebGPU] 2D-fold mul + permute dispatch (lift 65535 1D cap)

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Use requiredFeatures instance API on native + emscripten Dawn #20652
• -> [ExecuTorch][WebGPU] 2D-fold mul + permute dispatch (lift 65535 1D cap) #20651
• [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

Lift the 65535 workgroup-per-dim cap for mul and permute so they run at any numel.

mul.Tensor and permute still used compute_1d_workgroup_count, which throws once numel / wg_size > 65535 — hit by a realistic Llama-3.2-1B LoRA layer (mul over [2048, 8192] = 262k workgroups; permute of [2048, 2048] = 65536). add/sub/div/fill/sdpa already use the 2D fold; this brings mul + permute in line.

Key changes:

• mul/BinaryOp.cpp, permute/Permute.cpp — compute_1d_workgroup_count → compute_2d_workgroup_count (returns utils::WgCount); dispatch + resize hook now set both workgroup_count_x and workgroup_count_y.
• binary_mul.wgsl, permute.wgsl — main takes @builtin(num_workgroups); flat index gid.x + gid.y * (num_workgroups.x * wg_size) (regenerated *_wgsl.h).

Mirrors the landed add op fold (runtime/ops/add/{BinaryOp.cpp,binary_add.wgsl}).

Co-authored-with: Claude Code.

Differential Revision: D110149677

[pytorch-bot]

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❌ 1 New Failure

As of commit d29e08a with merge base 73c259e ():

NEW FAILURE - The following job has failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 7d8511c2be0f5b07b5e5f018b7eaa33f00bc98d4a2e46f299b55892a5e4af7ed /exec failed with exit code 139

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