add Count() to count items in the heap

source/tensor/XHeap.h

@@ -120,6 +120,9 @@ public:
 
     /* move item k up the tree */
     void Up(int k);
+
+    /* how many items are kept in the heap */
+    inline int Count() { return count; };
 };
 
 } /* end of the nts (NiuTrans.Tensor) namespace */