Fix constexpr in KernelRegistrationBuilder (#16906)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16906
In C++11, constexpr implies const, so these methods actually wouldn't be rvalue overloads as intended but const rvalue overloads.
Let's only apply the constexpr flag in C++14 to be safe.
Reviewed By: bddppq
Differential Revision: D13998486
fbshipit-source-id: a04d17ef0cc8f45e3d0a1ca9843d194f4f0f6f7f
