Arm Backend: Add tests for rsub.Scalar, transpose_copy.int and t_copy.default

[agrima1304]

These ops are not in the list of core ATen ops and are decomposed automatically when lowering the graph (https://docs.pytorch.org/docs/main/export.html#export-ir-decompositions), so only the tests need to be added.

All three of these ops are in this decomp table: https://github.com/pytorch/pytorch/blob/5d749ceb92c2c28bcfbdf918b4ab99b1a91fcb50/torch/_decomp/__init__.py#L295

Also adds a small comment in the sub op tests.

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14747

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 5 New Failures, 1 Cancelled Job, 1 Unrelated Failure

As of commit 2cd1d41 with merge base 418c584 ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 467bb7e2417442b0750f7e70bae2583e6b40e5ae5df26c9ff5f9c74ccd808fb7 /exec failed with exit code 1
• pull / unittest / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_linear_model
• Test CUDA Builds / test-voxtral-cuda-e2e / linux-job (gh)
  RuntimeError: Command docker exec -t a0ddd6c6f5c064c0c8999dacabd626766d8f2880017f2c09f9183c5f025abc16 /exec failed with exit code 2
• trunk / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (gh)
  RuntimeError: Command docker exec -t 8da51de781eebe55ed5d47e3235ceddb00e0461715e84afe43f974d17e26bf59 /exec failed with exit code 1
• trunk / test-custom-ops-macos (cmake) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

• trunk / test-models-macos-cpu (llama, portable) / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• Apple / build-demo-ios / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 74

This comment was automatically generated by Dr. CI and updates every 15 minutes.