title: Why does add_summary take an Integer for global_step rather than the natural TF variable global_step
body: <p>When using the tf.summary.Filewriter we can save the global step (to organize data in Tensorboard)</p>
  
  <pre><code>writer = tf.summary.FileWriter('directory',sess.graph)
  global_step = tf.Variable(0, name='global_step', trainable=False)
  
  % run some training, global_step iterated by calls to optimizer 
  
  writer.add_summary(summary,global_step=GLOBAL_STEP)
  </code></pre>
  
  <p>Why does add_summary need an GLOBAL_STEP to be an int rather than the tf variable global_step?  Since it the argument is explicitly named global_step, it seems like it is not meant to be used in a scenario where you have some step/count that is <em>not</em> the global step.  </p>
  
  <p>Is this just to save having to pass a session argument to the writer?  Why would you be running the writer outside of an environment with a running session?</p>