1. optimizer.minimize 与 global_step
optimizer = tf.train.**(learning_rate)
global_step = tf.Variable(0, trainable=False, name='global_step')
train_op = optimizer.minimize(loss, global_step=global_step)
optimizer = tf.train.**(learning_rate)
global_step = tf.Variable(0, trainable=False, name='global_step')
train_op = optimizer.minimize(loss, global_step=global_step)