测试GPU是否参与模型训练等运算

import tensorflow as tf
# Creates a graph.
a = tf.constant([6.0, 5.0, 3.0, 3.0, 2.0, 1.0], shape=[2, 3], name='a')
b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2], name='b