Fix unused variable warning in MultiLabelMarginCriterion.cu (#71594)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/71594
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D33692620
fbshipit-source-id: ae1ac664f1e7bce2ce054839b6d5eed4fdcd454f
(cherry picked from commit 4377a8b053a60d252cf49b5b92593f80ebb3af31)
