参考自https://stackoverflow.com/questions/41166681/what-does-global-step-mean-in-tensorflow
global_step记录的其实是train阶段每一步的索引，或者说是训练迭代的计数器，比如说在最后画loss和 accuracy的横坐标即是global_step
train_op = tf.train.GradientDescentOptimizer(learning_rate=LEARNING_RATE).minimize(loss_tensor,global_step=tf.train.create_global_step())
with tf.Session() as sess:
...
tf.logging.log_every_n(tf.logging.INFO,"np.mean(loss_evl)= %f at step %d",100,np.mean(loss_evl),sess.run(tf.train.get_global_step()))
INFO:tensorflow:np.mean(loss_evl)= 1.396970 at step 1
INFO:tensorflow:np.mean(loss_evl)= 1.221397 at step 101
INFO:tensorflow:np.mean(loss_evl)= 1.061688 at step 201
