[ExecuTorch][WebGPU] Add et_vk.apply_rotary_emb (interleaved RoPE) + ValueList multi-output

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Add et_vk.prepack (constant-tensor packing) for E2E weight loading #20265
• -> [ExecuTorch][WebGPU] Add et_vk.apply_rotary_emb (interleaved RoPE) + ValueList multi-output #20264
• [ExecuTorch][WebGPU] Add et_vk.embedding_q4gsw (4-bit groupwise-symmetric quantized embedding) #20263

Adds the WebGPU backend handler for et_vk.apply_rotary_emb.default (interleaved Llama rotary positional embedding) plus the ValueList graph-value support its multi-output signature requires.

The op rotates the query and key tensors by a shared freqs_cos/freqs_sin pair and is composed of two dispatches of one WGSL kernel: each thread handles one (even, odd) element pair of a head row (out[2i] = x[2i]*cos - x[2i+1]*sin, out[2i+1] = x[2i]*sin + x[2i+1]*cos), one dispatch writing xq_out and one writing xk_out, mirroring the Vulkan apply_rotary_emb reference (buffer-only, fp32, the interleaved .default variant). Each dispatch owns a distinct compute pipeline (the graph destructor releases per dispatch, so a shared handle would double-free); the workgroup size is a wg_size pipeline-override constant clamped to the device limit, both 1D dispatch counts go through WebGPUUtils::compute_1d_workgroup_count and are validated before any GPU-object allocation, and the embedded WGSL header is generated by gen_wgsl_headers.py.

The two outputs (xq_out, xk_out) are serialized by the Vulkan exporter as a single ValueList graph value, which the runtime did not previously model. This adds the ValueType::ValueList value kind, a value_lists_ table populated during build(), and a get_value_list accessor the handler uses to resolve the output ids. While in that code path it also closes a latent gap: a constant tensor whose constant_id is set but whose constants table is missing or out of range now throws (fail-loud) rather than silently leaving the buffer uninitialized.
@exported-using-ghexport

Differential Revision: D108428756

Differential Revision: D108428756

[pytorch-bot]

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