code: def adafactor_decay_rate_adam(beta2):
    
    t = tf.cast(tf.train.get_or_create_global_step(), tf.float32) + 1.0
    decay = beta2 * (1.0 - tf.pow(beta2, t - 1.0)) / (1.0 - tf.pow(beta2, t))
    return decay
docs: Second-moment decay rate like Adam, subsuming the correction factor.
  
    Args:
      beta2: a float between 0 and 1
    Returns:
      a scalar