message: Update links from official/mnist to official/r1/mnist.
diff: """tf.data.Dataset interface to the MNIST dataset.
  This is cloned from
  - https://github.com/tensorflow/models/blob/master/official/mnist/dataset.py
  + https://github.com/tensorflow/models/blob/master/official/r1/mnist/dataset.py
  """
  from __future__ import absolute_import
  