[ExecuTorch][WebGPU] Add rms_norm op

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Enable backend test suite + x86 CI #19960
• -> [ExecuTorch][WebGPU] Add rms_norm op #19959
• [ExecuTorch][WebGPU] Upload named-data constants in WebGPUGraph #19958

Adds the et_vk.rms_norm.default operator to the WebGPU backend: a WGSL compute shader using a cooperative tree reduction, one workgroup per row. The shader mirrors the Vulkan implementation (backends/vulkan/runtime/graph/ops/impl/RmsNorm.cpp, backends/vulkan/runtime/graph/ops/glsl/rms_norm_buffer.glsl); indexing assumes contiguous fp32 inputs. The handler fails loud (throws, mirroring Vulkan's VK_CHECK_COND) on invalid shape/dtype/dispatch-limit conditions, and defaults eps to the float32 machine epsilon.

The weight constant is uploaded via the named-data path added in the parent diff.

Differential Revision: D106887028

[pytorch-bot]

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