title: tf.data.Dataset.window example from the documentation fails
body: <p>I'm trying to use an example from the <a href="https://www.tensorflow.org/api_docs/python/tf/data/Dataset#window" rel="nofollow noreferrer">TF documentation</a> for <code>tf.data.Dataset.window</code> and the example from the documentation is failing.</p>
  
  <p>Code derived from the documentation:</p>
  
  <pre><code>import tensorflow as tf
  
  ds = tf.data.Dataset.range(7).window(2)
  next_element = ds.make_one_shot_iterator().get_next()
  
  with tf.Session() as sess:
      print(sess.run(next_element))
  </code></pre>
  
  <p>Produces this error (trace removed):</p>
  
  <pre><code>TypeError: Can not convert a _VariantDataset into a Tensor or Operation.
  During handling of the above exception, another exception occurred:
  TypeError: Fetch argument &lt;_VariantDataset shapes: (), types: tf.int64&gt; has invalid type &lt;class 'tensorflow.python.data.ops.dataset_ops._VariantDataset'&gt;, must be a string or Tensor. (Can not convert a _VariantDataset into a Tensor or Operation.)
  </code></pre>
  
  <p>So <code>iterator.get_next()</code> is returning a <code>VariantDataset</code> rather than the usual tensor.</p>
  
  <p><strong>TF Version: 1.13.1</strong></p>