[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]

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• [ROCm] MI350 CI jobs will have longer queue times due to CI migration

❌ 4 New Failures, 4 Unrelated Failures

As of commit 8f74aad with merge base 1b726b2 ():

NEW FAILURES - The following jobs have failed:

• pull / test-arm-backend-no-driver (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 7cf23458e6a9bd1ccf0b911f56596f4f58f64c38769c07120bdb2097e1a4b9df /exec failed with exit code 1
• pull / test-qnn-models-linux (dl3) / linux-job (gh)
  RuntimeError: Command docker exec -t e046b4f8f2e760e1748eee4be549a73d9f18d335b775a2dd5c40fffbefdb028c /exec failed with exit code 92
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 589666f5b735a644347bb280e3e389d8ffb996397155e76152fcecee4986bd1b /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5958c49b283a2b39fbf777232ca1be31b75adb73847a928664984ff578cfe509 /exec failed with exit code 1

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.arm64.2xlarge, executorch-ubuntu-22.04-... / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: received unexpected HTTP status: 503 Service Temporarily Unavailable.

BROKEN TRUNK - The following jobs failed but was 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)
• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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