[Int16] Add 16a8w support for avg_pool op

[Ninja91]

Stack from ghstack (oldest at bottom):

• -> [Int16] Add 16a8w support for avg_pool op #15585

Adds 16a8w quantized support for avg_pool op.

Differential Revision: D86236826

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15585

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❌ 2 New Failures, 6 Unrelated Failures

As of commit 3c9f1e7 with merge base 98afdeb ():

NEW FAILURES - The following jobs have failed:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh)
  RuntimeError: Command docker exec -t 109051909aae07d8d7d4a7918fb8b7e8bee257edfc1b3aa44d5a48776f77f0c3 /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-fvp (test_pytest_ops) / linux-job (gh)
  RuntimeError: Command docker exec -t cb14f0643c43fff4f40759000e536d8e46c66a05ec12eacfc03496bc7ea76c90 /exec failed with exit code 1

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

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / 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
• Test Metal Backend / export-voxtral-metal-artifact / macos-job (gh) (trunk failure)
  /Users/ec2-user/runner/_work/executorch/executorch/pytorch/executorch/pip-out/temp.macosx-11.0-arm64-cpython-311/cmake-out/configurations/optimized_native_cpu_ops_lib/NativeFunctions.h:72:228: error: no template named 'optional' in namespace 'torch::executor'; did you mean 'executorch::aten::optional'?

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