GemmKernel: Define static constexpr member variables out-of-class for C++14 compatibility
Description
GemmKernel: Define static constexpr member variables out-of-class for C++14 compatibility
In C++14, these variables need out-of-class definitions when they are ODR-used. Otherwise, there will be undefined symbols in the object file that cause link failures. This is fixed in C++17 (https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0386r2.pdf). GCC and clang backported the fix, but compilers like NVHPC didn't.