Dispose ExperimentResult objects at the end

Author: mstfbl

src/Microsoft.ML.AutoML/API/ExperimentResults/CrossValidationExperimentResult.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System;
 using System.Collections.Generic;
 using Microsoft.ML.Data;