Arm backend: Add docstrings to tosa/backend.py

[Sebastian-Larsson]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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

As of commit 586f31b with merge base 96189bf ():

NEW FAILURE - The following job has failed:

• trunk / test-models-macos-coreml (vit) / macos-job (gh)
  The process '/usr/bin/git' failed with exit code 128

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

• pull / test-binary-size-linux-gcc / linux-job (gh)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-setup-linux-gcc / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following jobs failed but was present on the merge base:

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

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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[zingo]

Got a test fail in
FAILED backends/arm/test/ops/test_floor_div.py::test_floor_divide_tosa_FP[op_floor_div_rank4_large_rand]
it has been fixed, rebasing to re-test with fix.

[zingo]

fails unrelated, only changes within files so buck2 build files should not need any update.