fix a bug

source/network/XNet.cpp

@@ -55,7 +55,7 @@ void XNetClearAll()
 XNet::XNet()
 {
     nodes.Clear();
-    isGradEfficient = false;
+    isGradEfficient = true;
 }
 
 /* de-constructor */
@@ -163,14 +163,13 @@ void XNet::Backward(XList &roots, XList &golds, LOSS_FUNCTION_NAME loss)
             BackwardNode(node, isGradEfficient); 
 
         if(isGradEfficient){
-            if(!XNoder::IsLeaf(node)){
             XLink & outgo = node->outgo;
             for(int i = 0; i < outgo.tailNum; i++){
                 XTensor * parent = outgo.tails[i];
                 ClearGrad(parent);
             }
-            }
-            else
+
+            if(XNoder::IsLeaf(node))
                 ClearGrad(node);
         }
     }