Fix gcc-5.4 compilation (#35935)
Summary:
It needs a hint how to hash `enum class` in `std::unordered_map`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35935
Test Plan: CI
Differential Revision: D20837750
Pulled By: malfet
fbshipit-source-id: 4208ee4bfa2e3cfbedf5b92bf18031225bf9dfa1
