[ONNX] Add quantization support to _avg_pool opset 9 and clean up (#79793)
- Add quantization support to _avg_pool opset 9
- Clean up reused / unused variables in avgpool helper
- Add types
- Sort `__all__`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79793
Approved by: https://github.com/BowenBao
