Fix unused variable warning in DepthwiseConv2d.cu (#71584)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71584
Test Plan: Sandcastle
Reviewed By: meyering
Differential Revision: D33692624
fbshipit-source-id: 6edba51a62e1754d9ef8ead4178688ada4bd6ffc
(cherry picked from commit 3f52e885fb1d620b3d5b200325e2bd6ea5f69d66)
