replace deprecated tf.initialize_all_variables

[DjangoPeng]

Replace deprecated tf.initialize_all_variables with tf.global_variables_initializer. The deprecated API has been removed after 2017-03-02. When users run the example, they would get the WARNING as below:

WARNING:tensorflow:From /usr/local/lib/python2.7/dist-packages/tensorflow/python/util/tf_should_use.py:175: initialize_all_variables (from tensorflow.python.ops.variables) is deprecated and will be removed after 2017-03-02.

It should be updated with the new and official API tf.global_variables_initializer.

[k8s-ci-robot]

Hi @DjangoPeng. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[jlewi]

Thanks

[jlewi]

/ok-to-test