[ExecuTorch][WebGPU] Add thin partitioner frontend

[JCNTH]

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• -> [ExecuTorch][WebGPU] Add thin partitioner frontend #21440

Give WebGPU a public frontend without changing delegation

WebGPU exports currently construct VulkanPartitioner directly, which obscures the intended frontend even though the runtime deliberately reuses Vulkan partitioning and serialization. This adds a composition-only WebGPUPartitioner and migrates the canonical WebGPU tester and documentation. It mirrors VulkanPartitioner behavior exactly.

Key changes:

• WebGPUPartitioner — forwards all constructor arguments, partition(), and ops_to_not_decompose() unchanged.
• WebGPU tester — uses the wrapper while preserving skip_bool_tensors=True and WEBGPU_SUPPORTED_OPS.
• Frontend contract tests — prove byte-identical serialized programs and the unchanged VulkanBackend delegate id.
• Documentation — presents the WebGPU-branded entry point and its policy-free boundary.

The invariant is unchanged: no runtime, serialization, backend-id, or capability-policy behavior changes.

Co-authored-with: Claude Code.

Differential Revision: D113924856

[pytorch-bot]

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