[ExecuTorch][WebGPU] aten.index.Tensor test suite (export + native golden)

[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

Adds the test suite for the aten.index.Tensor op (stacked on the op diff):

• test/ops/index/test_index.py: exports a module computing x[idx] through
  VulkanPartitioner for four configs (reorder/repeat indices over distinct self
  values, so a wrong-gather is visible), asserts a VulkanBackend delegate with
  index.Tensor absorbed (not a CPU fallback), and writes per-config .pte +
  .self/.idx/.golden.bin.
• test/native/test_index.cpp: a standalone Dawn binary that loads each .pte, feeds
  self (fp32) + index (int64 at the program boundary, narrowed to the int32 buffer)
  and compares the gather against the torch golden at 1e-3, with a single-output
  shape guard.
• Wired into CMake (webgpu_index_test), test/TARGETS (python_unittest test_index),
  and the Dawn native CI script.

Differential Revision: D109479000

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/20465

• 📄 Preview Python docs built from this PR

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❌ 3 New Failures, 1 Pending, 1 Unrelated Failure

As of commit f6f8a62 with merge base e03f777 ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh)
  RuntimeError: Command docker exec -t a2774aed3d3b7d3019c9b41035fa0b27d4661cee35b558a8896a9e5841b79ad4 /exec failed with exit code 92
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 8fbcdcec9677f9b118f239dbeafb874566a5d4ae5ad5c2f0cb536a066f1fd772 /exec failed with exit code 3
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script 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 / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh) (trunk failure)

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