[ExecuTorch][WebGPU] Add clone op (aten.clone.default)

[JulianCloudNTH]

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aten.clone.default is a pure flat copy on the buffer-only WebGPU backend, identical to view_copy: clone_impl reuses the existing add_flat_copy helper (output[i] = input[i]) and registers a handler under aten.clone.default. No new shader, generated WGSL header, or CMake source — it shares the view_copy flat-copy compute pipeline. Required for end-to-end Llama 3.2 1B (4-bit, KV cache): the exported model serializes 2 aten.clone.default ops into its runtime operator chain (the RoPE-frequency clones reused across all 16 transformer layers), so without a handler the partition graph-breaks at those nodes. Mirrors the Vulkan delegate, which registers the same op and routes a buffer clone to a flat view-copy.

Differential Revision: D109477717

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❌ 2 New Failures, 4 Unrelated Failures, 1 Unclassified Failure

As of commit 75f3e80 with merge base 1b726b2 ():

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• pull / unittest-editable / linux / linux-job (gh)
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• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)
• pull / unittest / macos / macos-job (gh) (trunk failure)
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• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
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