Cast return value of vector.at() to void to avoid nodiscard warning in MSVC. (#22061)
Summary:
Fix https://github.com/pytorch/pytorch/issues/22053
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22061
Differential Revision: D15957983
Pulled By: ezyang
fbshipit-source-id: e4416c5f0db2bc6b8bfaa27be52b942148ec7b3d