Add message to static_assert (#38519)
Summary:
From standard https://en.cppreference.com/w/cpp/language/static_assert, static_assert without message is not supported on C++ 14. Some compilers complained about this.
cc mcarilli
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38519
Differential Revision: D21589194
Pulled By: ezyang
fbshipit-source-id: d01555b2861703f0326a99bc5162e124695b0624
