* CHECK_XX in glog need the left and right values are both signed or
  unsigned integer. So we could use integer literal syntax in C/C++
  to make literal unsigned or signed. Add suffix 'UL' means unsigned
  long.

SparseGpuMatrix::mul(GpuMatrix, GpuMatrix),
CpuMatrix::mul(CpuSparseMatrix, CpuMatrix),
and GpuMatrix::mul(GpuSparseMatrix, GpuMatrix)

and GpuMatrix::mul(CpuMatrix, GpuSparseMatrix)