title: How to use several summary collections in Tensorflow?
body: <p>I have 2 distinctive groups of summaries. One is collected once per batch another one is collected once per epoch. How can I use <code>merge_all_summaries(key='???')</code> to collect summaries in this two groups separately? Doing it manually is always an option but there seems to be a better way.</p>
  
  <p>Illustration of how i think it should work:</p>
  
  <pre class="lang-py prettyprint-override"><code>      # once per batch 
        tf.scalar_summary("loss", graph.loss)
        tf.scalar_summary("batch_acc", batch_accuracy)
        # once per epoch
        gradients = tf.gradients(graph.loss, [W, D])
        tf.histogram_summary("embedding/W", W, collections='per_epoch')
        tf.histogram_summary("embedding/D", D, collections='per_epoch')
  
        tf.merge_all_summaries()                # -&gt; (MergeSummary...) :)
        tf.merge_all_summaries(key='per_epoch') # -&gt; NONE              :(
  </code></pre>