message: fixed calling Estimator '_distribution' method
  The code is calling a '_distribution' method which no longer exists.  It will produce error message <AttributeError: 'Estimator' object has no attribute '_distribution'> when training mirrored strategy with hooks.
  Changing '_distribution' to '_train_distribution' can fix the issue.
diff: @@ -1303,7 +1303,7 @@ class Estimator(object):
  # TODO(yuefengz): add a test for unwrapping per_device_hooks.
  def get_hooks_from_the_first_device(per_device_hooks):
  return [
  - self._distribution.unwrap(per_device_hook)[0]
  + self._train_distribution.unwrap(per_device_hook)[0]
  for per_device_hook in per_device_hooks
  ]
  