Adafactor with recommended learning rate schedule.

def afx_adafactor():
  """Adafactor with recommended learning rate schedule."""
  hparams = afx_adam()
  hparams.optimizer = "Adafactor"
  hparams.learning_rate_schedule = "rsqrt_decay"
  hparams.learning_rate_warmup_steps = 10000
  return hparams