Merge with ZHANG Yuhao branch (Don't use this! It's an incomplete version)

Minor error fixed.

source/network/XBackwardFunc.cpp

@@ -41,7 +41,6 @@ void XFuncGrad::MakeGrad(XTensor * node, bool isEfficient)
         CheckNTErrors(!node->isGrad || node->grad != NULL, "No gradient found!");
     }
 
-    //CheckNTErrors(node->grad != NULL, "No gradient found!");
     CheckNTErrors(income.tailNum == 1, "Too many input tensors for the function!");
 
     XTensor * input = income.tails[0];