fix return value issue for weight_variable and bias_variable.

Author: Oceania2018

src/TensorFlowNET.Core/APIs/tf.nn.cs

@@ -30,19 +30,21 @@ public static class nn
             public static Tensor conv2d(Tensor input, RefVariable filter, int[] strides, string padding, bool use_cudnn_on_gpu = true, 
                 string data_format= "NHWC", int[] dilations= null, string name = null)
             {
-                if (dilations == null)
-                    dilations = new[] { 1, 1, 1, 1 };
-
-                return gen_nn_ops.conv2d(new Conv2dParams
+                var parameters = new Conv2dParams
                 {
                     Input = input,
                     Filter = filter,
                     Strides = strides,
+                    Padding = padding,
                     UseCudnnOnGpu = use_cudnn_on_gpu,
                     DataFormat = data_format,
-                    Dilations = dilations,
                     Name = name
-                });
+                };
+
+                if (dilations != null)
+                    parameters.Dilations = dilations;
+
+                return gen_nn_ops.conv2d(parameters);
             }
 
             /// <summary>

test/TensorFlowNET.Examples/ImageProcess/DigitRecognitionCNN.cs

@@ -158,7 +158,6 @@ private Tensor conv_layer(Tensor x, int filter_size, int num_filters, int stride
                 layer += b;
                 return tf.nn.relu(layer);
             });
-
         }
 
         /// <summary>
@@ -195,7 +194,7 @@ private Tensor flatten_layer(Tensor layer)
             });
         }
 
-        private Tensor weight_variable(string name, int[] shape)
+        private RefVariable weight_variable(string name, int[] shape)
         {
             var initer = tf.truncated_normal_initializer(stddev: 0.01f);
             return tf.get_variable(name,
@@ -210,7 +209,7 @@ private Tensor weight_variable(string name, int[] shape)
         /// <param name="name"></param>
         /// <param name="shape"></param>
         /// <returns></returns>
-        private Tensor bias_variable(string name, int[] shape)
+        private RefVariable bias_variable(string name, int[] shape)
         {
             var initial = tf.constant(0f, shape: shape, dtype: tf.float32);
             return tf.get_variable(name,