Fix unused variable warning in ConvolutionMM2d.cu (#71593)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71593
Test Plan: Sancastle
Reviewed By: malfet
Differential Revision: D33692621
fbshipit-source-id: 64d079e10cdd9c2c71a42540fbf396f0f66efe11
(cherry picked from commit 6959ddd920ceae1d751665d7f48777a779fcbf23)
