fix compile error reported in issue #15911 (#15953)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15953
Fix issue reported in https://github.com/pytorch/pytorch/issues/15911
Reviewed By: csummersea
Differential Revision: D13633256
fbshipit-source-id: 3808f100ff7dedfe5e20708e72e6081ff07eb32c
