Upload named-data constants in WebGPUGraph

[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

The Vulkan serializer that the WebGPU backend reuses stores every non-empty constant in the PTE's named-data map with offset == UINT64_MAX and a named_key, rather than inline in the VK00 blob. WebGPUGraph::build previously handled only inline constants, so a delegated op's constant weights were never uploaded and the op produced all zeros. build now also fetches named-data constants via NamedDataMap::get_data, mirroring the path VulkanBackend already uses. aten.add was unaffected since it has no constant tensors; the first consumer is the rms_norm op in the child diff.

Differential Revision: D107288998

[pytorch-bot]

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