Arm backend: Add docstrings for operator_support/index_tensor_support.py

[Sebastian-Larsson]

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14505

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❌ 2 New Failures, 1 Cancelled Job

As of commit c89d869 with merge base 7e228ee ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 5ea0afb0645e803d50a3cb20407d0ee688d8bd7e71f1111856197e1571b22390 /exec failed with exit code 1
• trunk / test-arm-ootb-linux / linux-job (gh)
  RuntimeError: Command docker exec -t a7eb006475695a3a3ea8823f419cb3acfd94e9ae0a7155073804023986b9a9d5 /exec failed with exit code 1

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / test-models-macos-coreml (dl3) / macos-job (gh)
  ##[error]The operation was canceled.

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[zingo]

Arm oobt fail is unrelated and will be fixed by another PR