title: Where does next_batch in the TensorFlow tutorial batch_xs, batch_ys = mnist.train.next_batch(100) come from?
body: <p>I am trying out the TensorFlow tutorial and don't understand where does next_batch in this line come from? </p>
  
  <pre><code> batch_xs, batch_ys = mnist.train.next_batch(100)
  </code></pre>
  
  <p>I looked at </p>
  
  <pre><code>from tensorflow.examples.tutorials.mnist import input_data
  mnist = input_data.read_data_sets("MNIST_data/", one_hot=True)
  </code></pre>
  
  <p>And didn't see next_batch there either.</p>
  
  <p>Now when trying out next_batch in my own code, I am getting </p>
  
  <pre><code>AttributeError: 'numpy.ndarray' object has no attribute 'next_batch'
  </code></pre>
  
  <p>So I would like to understand where does next_batch come from? </p>