fix a small bug

source/network/XBackwardMath.cpp

@@ -91,7 +91,7 @@ dE/db = dE/dc * b.reduce(0,...,n-1,n+1,...) * \beta
 void XMathGrad::GradSumDim(XTensor * node)
 void XMathGrad::GradSumDim(XTensor * node)
 {
 {
     XLink &income = node->income;
     XLink &income = node->income;
-    CheckNTErrors(income.tailNum == 2, "Wrong input tensor number for SUM!");
+    CheckNTErrors(income.tailNum == 2, "Wrong input tensor number for SUMDIM!");
 
 
     XTensor * a = income.tails[0];
     XTensor * a = income.tails[0];
     XTensor * b = income.tails[1];
     XTensor * b = income.tails[1];