fix tf_text_graph_common tensor_content type bug

samples/dnn/tf_text_graph_common.py

@@ -324,6 +324,6 @@ def writeTextGraph(modelPath, outputPath, outNodes):
             for node in graph_def.node:
                 if node.op == 'Const':
                     if 'value' in node.attr and node.attr['value'].tensor.tensor_content:
-                        node.attr['value'].tensor.tensor_content = ''
+                        node.attr['value'].tensor.tensor_content = b''
 
         tf.train.write_graph(graph_def, "", outputPath, as_text=True)