title: cuda offset device pointer in host code
body: <p>I first process a matrix in cublas, I have already sent it to device and I want to process
  some column vector of the matrix, still use cublas function. I first try using pointer arithmetic operation to offset the device pointer from host, but it seems doesn't work.
  Is there any way I can process vector in matrix without copying it back to host.  </p>
  
  <p>for example: 
      cublasSscal (int n, float alpha, float *x, int incx); 
  is used to scale a vector 
      I have a device pointer point to a column major matrix B,
      i want scale the third column of B and without copy the vector back to host, how to do it?</p>