Updates beta_power variables every n batches and incrs counter.

def _finish(self, update_ops, name_scope):
    """Updates beta_power variables every n batches and incrs counter."""
    iter_ = self._get_iter_variable()
    beta1_power, beta2_power = self._get_beta_accumulators()
    with tf.control_dependencies(update_ops):
      with tf.colocate_with(iter_):

        def update_beta_op():
          update_beta1 = beta1_power.assign(
              beta1_power * self._beta1_t,
              use_locking=self._use_locking)
          update_beta2 = beta2_power.assign(
              beta2_power * self._beta2_t,
              use_locking=self._use_locking)
          return tf.group(update_beta1, update_beta2)
        maybe_update_beta = tf.cond(
            tf.equal(iter_, 0), update_beta_op, tf.no_op)
        with tf.control_dependencies([maybe_update_beta]):
          update_iter = iter_.assign(tf.mod(iter_ + 1, self._n_t),
                                     use_locking=self._use_locking)
    return tf.group(
        *update_ops + [update_iter, maybe_update_beta], name=name_scope)