[ExecuTorch][WebGPU] Add aten.index.Tensor (1D-self gather)

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• (to be filled)

Adds the WebGPU delegate handler for aten.index.Tensor, the 1D-self advanced-index
gather out[i] = self[index[i]] (output shape == index shape). This is the form the
VulkanPartitioner delegates -- it requires a 1D self and exactly one non-None index
(op_registry.py); 2D mask/freqs gathers stay on CPU. It mirrors the Vulkan delegate's
index_tensor op (IndexTensor.cpp + index_tensor_buffer.glsl) as a single compute
dispatch over the output elements, each reading the int32 index and gathering the
corresponding fp32 self element.

The op is composed as:

• index.wgsl: one workgroup-strided pass, out[i] = self[u32(index[i])], guarded by a
  numel bound; buffer-only, fp32 self/out, int32 index, 1D dispatch via the shared
  WebGPUUtils helpers (clamp workgroup size + 1D count).
• Index.cpp: validates the args (self/out tensors; indices ValueList with exactly one
  index tensor; fp32 self/out; int32 index; out numel == index numel), failing loud on
  any violation, then records the dispatch. row_width is dropped (always 1 for 1D self).

Differential Revision: D109478967

[pytorch-bot]

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❌ 6 New Failures, 1 Pending, 1 Unrelated Failure

As of commit 80e6fb0 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 6aafab84dd82b4ad1708290153820c460df540ae87154499c20e4f3228e54948 /exec failed with exit code 1
• pull / test-llama-runner-qnn-linux (fp32, qnn_16a16w, qnn) / linux-job (gh)
  RuntimeError: Command docker exec -t 6b1694091246206179ef2ce31c0e8259f5d90d8c8753680eafac8e8b7a621a57 /exec failed with exit code 137
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 7231e7aa96da0f5afb331d053510865d51263dc28f4515ed39411d21c184df28 /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 9c6c6673b752592e2dd91387f25cf3c659e1422b68261b020452a797bf4b36bc /exec failed with exit code 1
• pull / unittest-editable / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• Test QNN Backend / test-qnn / test-backend-linux (qnn, operators) / linux-job (gh)
  RuntimeError: Command docker exec -t 41e429dee959f4d46f0dcc15db994fd88870d1727efcbfaa7ad15737789c65d8 /exec failed with exit code 92

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 / 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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