Optimized save method

[JcmeLs]

This is not a issue，but a optimized save method.

1. import dependency

from tensorflow.python.framework.graph_util import convert_variables_to_constants 

2. Convert variables to constants and save

output_graph_def = convert_variables_to_constants(sess, sess.graph_def, output_node_names=
['output/outputs'])
    with tf.gfile.FastGFile('model/CTNModel.pb', mode='wb') as f:
        f.write(output_graph_def.SerializeToString())