In backprob of times Dense Sparse no switching to sparse

Source/ComputationNetworkLib/LinearAlgebraNodes.h

@@ -330,15 +330,8 @@ public:
 
         if (inputIndex == 0) // left derivative
         {
-			// TODO: we might leverage the fact that eg. for dense * sparse input(0)->Gradient() might ideally me block sparse.
-			// Currently AddMatrixProductOf(..) doesn't support that.
-
-			// currently we only support one combination when the input is sparse
-			// If input data is sparse, then gradient is block sparse.
-			// BUGBUG: This does not accumulate into the Input(0)->Gradient, which might cause problems elsewhere.
-			if (Input(1)->Value().GetMatrixType() == SPARSE && Input(0)->Gradient().GetMatrixType() == DENSE && Gradient().GetMatrixType() == DENSE)
-				Input(0)->Gradient().SwitchToMatrixType(SPARSE, MatrixFormat::matrixFormatSparseBlockCol, false);
-
+            // TODO: we might leverage the fact that eg. for dense * sparse input(0)->Gradient() might ideally be represented as block sparse.
+            // Currently AddMatrixProductOf(..) doesn't support that.
             auto input0Gradient = OneSampleTensorFor(0,  /*gradient=*/true,  fr.AllowBroadcast());
             auto input1         = OneSampleTensorFor(1,  /*gradient=*/false, fr.AllowBroadcast());
             auto outputGradient = OneSampleTensorFor(-1, /*gradient=*/true,  fr);

Source/Math/Matrix.cpp

@@ -4434,12 +4434,18 @@ void Matrix<ElemType>::MultiplyAndWeightedAdd(ElemType alpha, const Matrix<ElemT
             *c.m_GPUSparseMatrix = tmpSparse + (*c.m_GPUSparseMatrix) * beta;
             c.SetDataLocation(GPU, SPARSE);
         }
-		else if (a.m_matrixType == MatrixType::SPARSE && b.m_matrixType == MatrixType::SPARSE && c.m_matrixType == MatrixType::DENSE) // GPU, SPARSE * SPARSE -> DENSE
-			NOT_IMPLEMENTED;
-		else if (a.m_matrixType == MatrixType::SPARSE && b.m_matrixType == MatrixType::DENSE && c.m_matrixType == MatrixType::SPARSE) // GPU, SPARSE * DENSE -> SPARSE
-			NOT_IMPLEMENTED;
-		else // No combination left
-            NOT_IMPLEMENTED; // We should never get here.
+        else if (a.m_matrixType == MatrixType::SPARSE && b.m_matrixType == MatrixType::SPARSE && c.m_matrixType == MatrixType::DENSE) // GPU, SPARSE * SPARSE -> DENSE
+        {
+            NOT_IMPLEMENTED;
+        }
+        else if (a.m_matrixType == MatrixType::SPARSE && b.m_matrixType == MatrixType::DENSE && c.m_matrixType == MatrixType::SPARSE) // GPU, SPARSE * DENSE -> SPARSE
+        {
+            NOT_IMPLEMENTED;
+        }
+        else // No combination left
+        {
+            NOT_IMPLEMENTED;
+        }
     }
 }