Second-moment decay rate like Adam, subsuming the correction factor.

  Args:
    beta2: a float between 0 and 1
  Returns:
    a scalar

def adafactor_decay_rate_adam(beta2):
  """Second-moment decay rate like Adam, subsuming the correction factor.

  Args:
    beta2: a float between 0 and 1
  Returns:
    a scalar
  """
  t = tf.to_float(tf.train.get_or_create_global_step()) + 1.0
  decay = beta2 * (1.0 - tf.pow(beta2, t - 1.0)) / (1.0 - tf.pow(beta2, t))
  # decay = tf.cond(tf.equal(t, 1.0), lambda: beta2, lambda: decay)
  return decay