程序在需要读取多幅图片时，
需要使用多线程来进行操作：
coord = tf.train.Coordinator()
threads = tf.train.start_queue_runners(coord=coord)
# use multithreading to output image_batch and label_batch
try:
while not coord.should_stop() and i < 1:
image_batch, label_batch = sess.run([image_batch, label_batch])
# just test one batch
for j in np.arange(FLAGS.train_batch_size):
print('label: %d' % label_batch[j])
# plt.imshow(image_batch[j, :, :, :])
# plt.show()
i += 1
except tf.errors.OutOfRangeError:
print('done!')
finally:
coord.request_stop()
coord.join(threads)
