make XTensor::GetDim support negtive index

source/tensor/XTensor.cpp

@@ -427,7 +427,11 @@ int XTensor::GetDim(const int dim)
 {
 {
     CheckNTErrors(dim < order, "dimenision is out of range!");
     CheckNTErrors(dim < order, "dimenision is out of range!");
     
     
-    return dimSize[dim];
+    int d = dim;
+    if(dim < 0)
+        d = order - 1;
+
+    return dimSize[d];
 }
 }
 
 
 /* 
 /*