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

reference or raw matrix (Matrix & or Matrix *)
contextProjectionForward
contextProjectionBackward
contextProjectionBackwardData
contextProjectionBackwardWeight
classificationError
The mul functions would be updated later.

* The original SparseRowMatrix use two fields to store each rows,
  which let code very confusing. Try to extract a RowBuffer class,
  for SparseRowMatrix data storage, and manage auto-growth logic.