fix build error

Author: hcur

src/TensorFlowNET.Core/Graphs/Graph.cs

@@ -17,6 +17,7 @@ limitations under the License.
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.Collections.Specialized;
 using System.Linq;
 using System.Runtime.InteropServices;
 using static Tensorflow.Binding;