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

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] aten.index.Tensor test suite (export + native golden) #20465
• [ExecuTorch][WebGPU] Add aten.index.Tensor (1D-self gather) #20464
• -> [ExecuTorch][WebGPU] Add clone op (aten.clone.default) #20463
• [ExecuTorch][WebGPU] Flatten landed-op test dirs to test/ops/test_<op>.py #20435
• [ExecuTorch][WebGPU] cat op test suite (cases.py op-test framework) #20399
• [ExecuTorch][WebGPU] Add cat op + ValueList graph support (aten.cat.default) #20398
• [ExecuTorch][WebGPU] permute_copy op test suite (cases.py op-test framework) #20397
• [ExecuTorch][WebGPU] Add permute_copy + IntList graph support (aten.permute_copy.default) #20396
• [ExecuTorch][WebGPU] slice_copy op test suite (cases.py op-test framework) #20395
• [ExecuTorch][WebGPU] Add slice_copy op (aten.slice_copy.Tensor) #20394
• [ExecuTorch][WebGPU] squeeze_copy + unsqueeze_copy test suites (cases.py op-test framework) #20393
• [ExecuTorch][WebGPU] Add squeeze_copy + unsqueeze_copy (flat copies) #20392
• [ExecuTorch][WebGPU] sigmoid op test suite (cases.py op-test framework) #20391
• [ExecuTorch][WebGPU] Add sigmoid op (aten.sigmoid.default) #20390
• [ExecuTorch][WebGPU] select_copy op test suite (cases.py op-test framework) #20363
• [ExecuTorch][WebGPU] Add select_copy op (aten.select_copy.int) #20362
• [ExecuTorch][WebGPU] view_copy op test suite (cases.py op-test framework) #20361
• [ExecuTorch][WebGPU] Add view_copy op (aten.view_copy.default) #20360
• [ExecuTorch][WebGPU] mul op test suite (cases.py op-test framework) #20359

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

[pytorch-bot]

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❌ 2 New Failures, 2 Pending, 1 Unrelated Failure

As of commit 3ea2424 with merge base e03f777 ():

NEW FAILURES - The following jobs have failed:

• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 75c1b0d1084e901ce39a5db0a702db794d12ff7d5a12220ba137329686784d88 /exec failed with exit code 137
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3ad942abbdaf1c9dcc01c8ba6cf52e50cb780785da6bb85f400009cd70e23199 /exec failed with exit code 3

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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