Function std::cuda_half_gemm

int cuda_half_gemm(
    struct CudaHalfBuf a,
    int ta,
    struct CudaHalfBuf b,
    int tb,
    struct CudaHalfBuf c,
    unsigned long m,
    unsigned long n,
    unsigned long k,
)

c[m,n] = op(a)[m,k] * op(b)[k,n] (ta/tb as in cuda_gemm), a and b of the same dtype; c in that dtype (_relu: then max(0, .) in the epilogue), or fp32 with _f32.