Arm backend: Add docstrings for operator_support/reduce_sum_support.py

[Sebastian-Larsson]

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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❌ 1 New Failure, 4 Cancelled Jobs, 3 Unrelated Failures

As of commit d98148d with merge base acf5b4b ():

NEW FAILURE - The following job has failed:

• Apple / build-frameworks-ios / macos-job (gh)
  No files were found with the provided path: /Users/runner/work/_temp/artifacts/. No artifacts will be uploaded.

CANCELLED JOBS - The following jobs were cancelled. Please retry:

• pull / test-binary-size-linux-gcc / linux-job (gh)
• pull / test-openvino-linux / linux-job (gh)
• pull / test-setup-linux-gcc / linux-job (gh)
• trunk / test-llama-runner-mac (fp32, coreml) / macos-job (gh)
  ##[error]The operation was canceled.

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

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

• 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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