[ExecuTorch][WebGPU] et_vk.sdpa channel-attention tests

[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

Splits the channel-attention routing case out of the et_vk.sdpa per-entry-QK op diff into its own test diff, stacked directly above it (op below, tests above). Adds the chattn_davit case to the et_vk_sdpa suite in test/op_tests/cases.py, exercising the per-entry QK kernel path (num_rows below the per-row floor).

Co-authored-with: Claude Code.

Differential Revision: D111072706

[pytorch-bot]

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