add back Graph.IEnumerable<Operation>

Author: Oceania2018

src/TensorFlowNET.Core/Graphs/Graph.cs

@@ -75,7 +75,10 @@ all variables that are created during the construction of a graph. The caller
     ///     then create a TensorFlow session to run parts of the graph across a set of local and remote devices.
     /// </summary>
     /// <remarks>https://www.tensorflow.org/guide/graphs <br></br>https://www.tensorflow.org/api_docs/python/tf/Graph</remarks>
-    public partial class Graph : DisposableObject//, IEnumerable<Operation>
+    public partial class Graph : DisposableObject,
+#if !SERIALIZABLE
+        IEnumerable<Operation>
+#endif
     {
         private Dictionary<int, ITensorOrOperation> _nodes_by_id;
         public Dictionary<string, ITensorOrOperation> _nodes_by_name;
@@ -524,17 +527,19 @@ public override string ToString()
             }
 
             return debugString;*/
-        }
-
-        /*private IEnumerable<Operation> GetEnumerable()
+        }
+
+#if !SERIALIZABLE
+        private IEnumerable<Operation> GetEnumerable()
             => c_api_util.tf_operations(this);
 
         IEnumerator<Operation> IEnumerable<Operation>.GetEnumerator()
             => GetEnumerable().GetEnumerator();
 
         IEnumerator IEnumerable.GetEnumerator() 
-            => throw new NotImplementedException();*/
-
+            => throw new NotImplementedException();
+#endif
+
         public static implicit operator IntPtr(Graph graph)
         {
             return graph._handle;