fix _assign_new_value for BatchNormaliztion.

Author: Oceania2018

src/TensorFlowNET.Core/Gradients/TapeTensor.cs

@@ -22,5 +22,8 @@ public Tensor ZerosLike()
 
         public Tensor OnesLike()
             => tf.ones(shape: shape, dtype: dtype);
+
+        public override string ToString()
+            => $"{id}, {shape}, {dtype.as_numpy_name()}";
     }
 }

src/TensorFlowNET.Keras/Layers/BatchNormalization.cs

@@ -196,7 +196,7 @@ private Tensor _fused_batch_norm(Tensor inputs, Tensor training)
                     _assign_moving_average(moving_variance, variance, momentum_tensor);
 
                 if (use_fused_avg_updates)
-                    _assign_new_value(moving_variance, mean);
+                    _assign_new_value(moving_variance, variance);
                 else
                     _assign_moving_average(moving_variance, variance, momentum_tensor);