title: Display log into console with tf_logging
body: <p>I'm playing with <a href="https://github.com/tensorflow/tensorflow/blob/master/tensorflow/contrib/slim/python/slim/learning.py" rel="nofollow">slim.learning.train</a> and want to display the log in the console. According to the source code this is done using <code>tf_logging</code> module :</p>
  
  <pre><code>if 'should_log' in train_step_kwargs:
      if sess.run(train_step_kwargs['should_log']):
        logging.info('global step %d: loss = %.4f (%.2f sec)',
                     np_global_step, total_loss, time_elapsed) 
  </code></pre>
  
  <p>I can run my training loop but there's no logs in the console. How can I enable it ?</p>