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]

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❌ 2 New Failures

As of commit 9e43284 with merge base 45336ce ():

NEW FAILURES - The following jobs have failed:

• pull / test-samsung-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t e0312b859d021380148a5031ece66c80c9ceddf7a7b01f6cead209a0e3430992 /exec failed with exit code 1
• Test CUDA Builds / export-voxtral-cuda-artifact / linux-job (gh)
  RuntimeError: Command docker exec -t 9c9cf3a335a77d96f61fda58cf0c39adbc4bfcf3ab680e71d3def6be7dd83943 /exec failed with exit code 2

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[Sebastian-Larsson]

Unrelated CI errors.