Move the functions operating on fp16 out of the std namespace and into the Eigen::numext namespace
Move the functions operating on fp16 out of the std namespace and into the Eigen::numext namespace

ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,pasuka/eigen,pasuka/eigen,ritsu1228/eigen,pasuka/eigen,ROCmSoftwarePlatform/hipeigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen