Fix unused variable warning in FractionalMaxPool3d (#71591)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71591
Test Plan: Sandcastle
Reviewed By: ngimel
Differential Revision: D33692623
fbshipit-source-id: f96446751f41555a6b5e5289f94efb98c87c66d0
(cherry picked from commit a5c272793e55c4a41af553727a97f490c8e152fa)
