title: tf.data.Dataset: how to get the dataset size (number of elements in an epoch)?
body: <p>Let's say I have defined a dataset in this way:</p>
  
  <pre><code>filename_dataset = tf.data.Dataset.list_files("{}/*.png".format(dataset))
  </code></pre>
  
  <p>how can I get the number of elements that are inside the dataset (hence, the number of single elements that compose an epoch)?</p>
  
  <p>I know that <code>tf.data.Dataset</code> already knows the dimension of the dataset, because the <code>repeat()</code> method allows repeating the input pipeline for a specified number of epochs. So it must be a way to get this information.</p>