[ExecuTorch][WebGPU] Add GPU timestamp-query profiling (WebGPUQueryPool)

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> [ExecuTorch][WebGPU] Add GPU timestamp-query profiling (WebGPUQueryPool) #20167
• [ExecuTorch][WebGPU] SDPA test suite: replay + dynamic input_pos + in-graph KV cache #20087
• [ExecuTorch][WebGPU] Add fused SDPA (sdpa_with_kv_cache) with dynamic input_pos #20086
• [ExecuTorch][WebGPU] SymInt live-scalar mechanism + et_vk.select_as_symint #20085
• [ExecuTorch][WebGPU] Add update_cache tests (native numeric + export) #20084
• [ExecuTorch][WebGPU] Add update_cache op (llama.update_cache) #20083
• [ExecuTorch][WebGPU] Add per-pass dispatch ordering + scratch buffer tests #20080
• [ExecuTorch][WebGPU] Switch native backend from wgpu-native to Dawn (Tint) + SwiftShader #20079

Add a faithful re-port of Vulkan's vkapi::QueryPool (backends/vulkan/runtime/vk_api/QueryPool.{h,cpp}) so a bench can read true on-GPU per-kernel time, isolated from submit/readback latency — the basis for comparing the WGSL SDPA kernels against the Vulkan reference. Opt-in via the WEBGPU_TIMESTAMP_QUERY env var; off by default, so the production execute() path is byte-identical.

WebGPUQueryPool mirrors the Vulkan ShaderDuration data model and the ticks->ns conversion exactly. Three deviations are forced by the WebGPU API (not unforced divergences): per-dispatch bracketing uses a compute-pass timestampWrites descriptor (begin/end-of-pass) since WebGPU has no mid-encoder writeTimestamp; results are read via resolveQuerySet + buffer map (no host-side vkGetQueryPoolResults); and the TimestampQuery capability is requested as an explicit device feature (fail-open if the adapter lacks it). WebGPUGraph::execute() brackets each compute pass when the pool is active; chained update_cache/QK/softmax/AV dispatches carry a kernel_name label for attribution.

Co-authored-with Claude.
@exported-using-ghexport

Differential Revision: D107678235

Differential Revision: D107678235

[pytorch-bot]

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