message: Delete confusing comment related to missing output Variable
  The method KMeans().training_graph() does not return anymore the
  output Variable cluster_centers_var. Thus the related comment
  is outdated and should be deleted.
diff: @@ -188,7 +188,6 @@ class _ModelFn(object):
  # center.
  # is_initialized: scalar indicating whether the initial cluster centers
  # have been chosen; see init_op.
  - # cluster_centers_var: a Variable containing the cluster centers.
  # init_op: an op to choose the initial cluster centers. A single worker
  # repeatedly executes init_op until is_initialized becomes True.
  # training_op: an op that runs an iteration of training, either an entire
  