Calculate learning rate with linear warmup and rsqrt decay.

def get_learning_rate(learning_rate, hidden_size, learning_rate_warmup_steps):
  """Calculate learning rate with linear warmup and rsqrt decay."""
  with tf.name_scope("learning_rate"):
    warmup_steps = tf.to_float(learning_rate_warmup_steps)
    step = tf.to_float(tf.train.get_or_create_global_step())

    learning_rate *= (hidden_size ** -0.5)
    # Apply linear warmup
    learning_rate *= tf.minimum(1.0, step / warmup_steps)
    # Apply rsqrt decay
    learning_rate *= tf.rsqrt(tf.maximum(step, warmup_steps))

    # Save learning rate value to TensorBoard summary.
    tf.summary.scalar("learning_rate", learning_rate)

    return learning_rate