Arm backend: Align tosa backend, partitioner and mapping with doc style

[Sebastian-Larsson]

While the files mostly had docstrings, they did not adhere to the docstring style in the rest of the arm backend. Align them with this patch.

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/15827

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As of commit 5af4099832e28fed9f1cb87652df7b8763839942 with merge base b1e3e28bb611e06d484138be27221faffd89f565 (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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[zingo]

Good to merge but got Arm test error trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (push) trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (push)

[zingo]

Seem to be timeout, rerun test to check

[zingo]

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

[zingo]

Rebased after problem PR was reverted