Fix unused variable warning in AveragePool2d (#71585)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71585
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D33692619
fbshipit-source-id: 6f851a0925e7b91e107aa35353986a1fcaf630c4
(cherry picked from commit 53ce218424b84d9cf5aa25908afbacaf1f5ac757)
