Fix unordered-map-over-enum for GCC 5.4 (#41063)
Summary:
Forgot to add this to https://github.com/pytorch/pytorch/pull/41055
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41063
Differential Revision: D22407451
Pulled By: malfet
fbshipit-source-id: 6f06653b165cc4817d134657f87caf643182832a
