Arm backend: Add docstrings for tosa/dialect/lib.py

[Sebastian-Larsson]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15828

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As of commit 058df9e90b0eab54c17a7176895b03429af63aee with merge base 3bbe1730f3c70d8fb24cbd14fde7a8540e949385 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[zingo]

Timeout again :( trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (push)Cancelled after ...

[zingo]

Seem to be happening after https://github.com/pytorch/executorch/pull/14109 was merged, trying to revert that PR