[ExecuTorch][WebGPU] et_vk.apply_rotary_emb_hf: HuggingFace rotate-half RoPE runtime op (unblocks Qwen3)

[JCNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] apply_rotary_emb_hf op tests #20876
• -> [ExecuTorch][WebGPU] et_vk.apply_rotary_emb_hf: HuggingFace rotate-half RoPE runtime op (unblocks Qwen3) #20875
• [ExecuTorch][WebGPU] et_vk conv2d im2col-GEMM tests #20874
• [ExecuTorch][WebGPU] et_vk conv2d: route standard (groups==1) conv through an im2col tiled GEMM (1.1-2.4x) #20873
• [ExecuTorch][WebGPU] et_vk.sdpa channel-attention tests #20872
• [ExecuTorch][WebGPU] et_vk.sdpa: shape-route QK to a per-entry kernel for channel attention (15-30x faster) #20871
• [ExecuTorch][WebGPU] Migrate quantized_linear to make_compute_pipeline #20870
• [ExecuTorch][WebGPU] Migrate multi-dispatch ops to make_compute_pipeline #20869
• [ExecuTorch][WebGPU] Migrate 9 ops to make_compute_pipeline (layout:"auto") #20868
• [ExecuTorch][WebGPU] Adopt WebGPU layout:"auto" in make_compute_pipeline #20867
• [ExecuTorch][WebGPU] slice_copy native golden test #20866
• [ExecuTorch][WebGPU] slice_copy Double-arg scalar handling + view_copy/alias glue #20865
• [ExecuTorch][WebGPU] relu op tests #20864
• [ExecuTorch][WebGPU] Add relu op (shared unary handler; sigmoid adopts make_compute_pipeline) #20863
• [ExecuTorch][WebGPU] max_pool2d op tests #20862
• [ExecuTorch][WebGPU] Add max_pool2d op #20861
• [ExecuTorch][WebGPU] upsample_nearest2d op tests #20860
• [ExecuTorch][WebGPU] Add upsample_nearest2d op #20859
• [ExecuTorch][WebGPU] constant_pad_nd op tests #20858
• [ExecuTorch][WebGPU] Add constant_pad_nd op #20857
• [ExecuTorch][WebGPU] addmm op tests #20856
• [ExecuTorch][WebGPU] Add optimized addmm op #20855
• [ExecuTorch][WebGPU] embedding op tests #20854
• [ExecuTorch][WebGPU] Add embedding op #20853
• [ExecuTorch][WebGPU] SDPA op tests #20852
• [ExecuTorch][WebGPU] Add optimized SDPA op #20851
• [ExecuTorch][WebGPU] conv2d op tests #20850
• [ExecuTorch][WebGPU] Add optimized conv2d op #20849
• [ExecuTorch][WebGPU] linear_fp32 op tests #20848
• [ExecuTorch][WebGPU] Add optimized linear_fp32 op #20847
• [ExecuTorch][WebGPU] layer_norm op tests #20846
• [ExecuTorch][WebGPU] Add optimized layer_norm op #20845
• [ExecuTorch][WebGPU] gelu op tests #20844
• [ExecuTorch][WebGPU] Add optimized gelu op #20843
• [ExecuTorch][WebGPU] Shared pipeline + dispatch-grid + vec4-align helpers (op-suite foundation) #20842

Problem: the WebGPU runtime registers only et_vk.apply_rotary_emb (the interleaved/Meta RoPE convention). HuggingFace-derived models (Qwen3, etc.) export the rotate-half convention, which fuses under VulkanPartitioner into et_vk.apply_rotary_emb_hf — an op the runtime graph builder has no handler for, so WebGPUGraph::build() throws and the delegate is rejected at load with DelegateInvalidCompatibility (et_load error 48). The whole model then fails to load on WebGPU.

Solution: add the et_vk.apply_rotary_emb_hf runtime kernel + handler as a rotate-half sibling of the interleaved op.

Before: only apply_rotary_emb (interleaved) is registered; HF-RoPE models throw at load.
After: both conventions are handled; HF-RoPE models (Qwen3) load and run.

Implementation:

• New rotary_embedding_hf.wgsl: one thread per (i, i+half_dim) pair (rotate-half pairing vs the interleaved even/odd), reading a full [max_seq, rotary_dim] freqs table indexed at row start_pos + s. Scalar, wg_size 64 — structural + optimization parity with the interleaved kernel (RoPE is ~1% of runtime; vec4 is neutral for this elementwise-class op on Apple's scalar ALU).
• RotaryEmbedding.cpp: apply_rotary_emb_hf_impl mirrors the interleaved handler; it parses the extra start_pos arg as a build-time Int (baked) or a runtime SymInt (dynamic KV-cache decode) exactly as Sdpa.cpp handles input_pos, and registers a seq resize hook (xq/xk) plus a start_pos resize hook (dynamic decode). Full rotary only (rotary_dim == head_dim); partial-rotary passthrough throws (documented follow-up; Qwen3 uses full RoPE). Mirrors Vulkan et_vk.apply_rotary_emb_hf (backends/vulkan/runtime/graph/ops/impl/RotaryEmbedding.cpp).
• Registers et_vk.apply_rotary_emb_hf.default.

Constraints: full rotary only for now; scalar one-thread-per-pair, kept at parity with the interleaved sibling rather than vec4 (neutral for RoPE per the closed vec4 sweep).

Co-authored-with: Claude Code.

Differential Revision: D111009173

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