Enable backend test suite + x86 CI

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• -> Enable backend test suite + x86 CI #19957
• Add rms_norm op #19956
• Upload named-data constants in WebGPUGraph #19955

Wires the WebGPU backend into the standard ExecuTorch backend test suite and adds an x86 Linux CI job, mirroring the Vulkan delegate: backends/test/suite/flows/webgpu.py plus a WebGPUTester, run by oss/.github/workflows/test-backend-webgpu.yml on SwiftShader (a software Vulkan adapter, via wgpu-native, minimal dependencies, no GPU).

Two fixes were needed for SwiftShader's downlevel limits: request the adapter's full requiredLimits at device creation (software adapters default storage-buffer limits to 0), and make the add op's workgroup size dynamic instead of a hardcoded constant. The WGSL now declares a pipeline-overridable override wg_size: u32 = 256 and the host clamps it to the device's maxComputeInvocationsPerWorkgroup (256 on real GPUs and lavapipe, 128 on SwiftShader), so SwiftShader's 128-invocation cap no longer forces a smaller workgroup size on real hardware. This mirrors the dynamic-workgroup-sizing approach in D107259348 and opens the door to selecting device/algorithm-optimal sizes later. The add op also validates its 1D dispatch count (capped at 65535) before allocating any GPU objects. The editable CMake build additionally marks the vulkan_schema subdirectory EXCLUDE_FROM_ALL so the WebGPU ALL build does not pull in targets that need glslc.

Differential Revision: D107288999

[pytorch-bot]

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