Learning rate warmup multiplier.

def _learning_rate_warmup(warmup_steps, warmup_schedule="exp", hparams=None):
  """Learning rate warmup multiplier."""
  if not warmup_steps:
    return tf.constant(1.)

  tf.logging.info("Applying %s learning rate warmup for %d steps",
                  warmup_schedule, warmup_steps)

  warmup_steps = tf.to_float(warmup_steps)
  global_step = _global_step(hparams)

  if warmup_schedule == "exp":
    return tf.exp(tf.log(0.01) / warmup_steps)**(warmup_steps - global_step)
  else:
    assert warmup_schedule == "linear"
    start = tf.constant(0.35)
    return ((tf.constant(1.) - start) / warmup_steps) * global_step + start