make XTensor::GetDim support negtive index

source/tensor/XTensor.cpp

@@ -426,8 +426,12 @@ get the size of a given dimension
 int XTensor::GetDim(const int dim)
 {
     CheckNTErrors(dim < order, "dimenision is out of range!");
+    
+    int d = dim;
+    if(dim < 0)
+        d = order - 1;
 
-    return dimSize[dim];
+    return dimSize[d];
 }
 
 /*