Fix return-type-is-always-copy warning (#47279)
Summary:
`std::vector<bool>` can not return values by reference, since they are stored as bit fields
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47279
Reviewed By: glaringlee
Differential Revision: D24705188
Pulled By: malfet
fbshipit-source-id: 96e71cc4b9881f92af3b4a508d397deab6d68174
