[ExecuTorch][WebGPU] 2D compute dispatch — lift the 65535 per-dim cap (prefill path)

[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

Lift the 65535 workgroup-per-dim dispatch cap so single-shot SDPA prefill runs at any sequence length.

Problem: The WebGPU backend is 1D-dispatch-only and throws when a kernel's workgroup count exceeds the device per-dim limit (maxComputeWorkgroupsPerDimension, spec floor 65535). SDPA prefill QK exceeds it around S~362 (softmax/AV at S=2048), blocking single-shot / long-context prefill.

Solution: Fold a >limit 1D workgroup count into 2D; the shader reconstructs the linear index from @builtin(num_workgroups).

• Before: compute_1d_workgroup_count throws if count > limit; dispatch (count, 1, 1).
• After: compute_2d_workgroup_count returns {count, 1} (fast path) or {limit, div_up(count, limit)}; dispatch (x, y, 1).

Implementation:

• WgCount + pure fold_workgroup_count_2d + compute_2d_workgroup_count in WebGPUUtils.h (device-free, unit-testable; queried_max_workgroups factored out of the 1D path)
• WebGPUDispatch.workgroup_count_y (default 1, declared last so existing aggregate inits are unchanged); both dispatchWorkgroups calls + the profiling record pass (x, y, 1)
• Per-kernel in-shader reconstruction: thread-form idx = gid.x + gid.y*(num_workgroups.x*wg_size) (QK/AV/add); row-form row_idx = wid.x + wid.y*num_workgroups.x (softmax — keeps a valid predicate, not an early return, so workgroupBarrier()s stay uniform)
• Sdpa.cpp: QK/softmax/AV counts via the 2D helper; the dynamic-input_pos resize hook recomputes both x and y for QK
• Mirrors Vulkan dispatch (Vulkan itself does not guard the per-dim limit)

Constraints:

• y=1 fast path keeps every non-folded dispatch byte-identical to the prior 1D path
• Scope = prefill path only; rms_norm/embedding/lm_head/update_cache are row/token-indexed and never hit the cap, so they keep the 1D path
• Throws if a 3rd dispatch dimension would be needed — unreachable for real prefill (the uint32 element guard fires first at S~11585)

Co-authored-with: Claude Code.

Differential Revision: D109517684

[pytorch-bot]

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❌ 1 New Failure

As of commit d54c2d2 with merge base 55a71e6 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner (gh)
  >>> Lint for backends/webgpu/runtime/ops/add/BinaryOp.cpp:

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