fix compile issue.

Author: Oceania2018

src/TensorFlowNET.Core/Sessions/_ElementFetchMapper.cs

@@ -11,9 +11,9 @@ namespace Tensorflow
     public class _ElementFetchMapper<T> : _FetchMapper<T>
     {
         private List<object> _unique_fetches = new List<object>();
-        private Func<List<object>> _contraction_fn;
+        private Func<List<object>, NDArray> _contraction_fn;
 
-        public _ElementFetchMapper(List<T> fetches, Func<List<object>> contraction_fn)
+        public _ElementFetchMapper(List<T> fetches, Func<List<object>, NDArray> contraction_fn)
         {
             foreach(var fetch in fetches)
             {

test/TensorFlowNET.UnitTest/VariableTest.cs

@@ -38,7 +38,7 @@ public void Add()
                 session.run(model);
                 for(int i = 0; i < 5; i++)
                 {
-                    //x = x + 1;
+                    // x = x + 1;
                     var result = session.run(x);
                     print(result);
                 }