Fix model.evaluate don't have output

Author: Wanglongzhi2001

src/TensorFlowNET.Core/Keras/Engine/ICallback.cs

@@ -13,5 +13,6 @@ public interface ICallback
     void on_predict_batch_end(long end_step, Dictionary<string, Tensors> logs);
     void on_predict_end();
     void on_test_begin();
+    void on_test_batch_begin(long step);
     void on_test_batch_end(long end_step, IEnumerable<(string, Tensor)> logs);
 }

src/TensorFlowNET.Keras/Engine/Model.Evaluate.cs

@@ -72,12 +72,13 @@ public void evaluate(NDArray x, NDArray y,
 
                 foreach (var step in data_handler.steps())
                 {
-                    callbacks.on_train_batch_begin(step);
+                    callbacks.on_test_batch_begin(step);
                     logs = test_function(data_handler, iterator);
                     var end_step = step + data_handler.StepIncrement;
                     callbacks.on_test_batch_end(end_step, logs);
                 }
             }
+            Console.WriteLine();
             GC.Collect();
             GC.WaitForPendingFinalizers();
         }