Compute the length of each sequence in the batch.

  Args:
    emb: a sequence embedding Tensor with shape [batch, max_time, 1, depth].
  Returns:
    a Tensor with shape [batch].

def length_from_embedding(emb):
  """Compute the length of each sequence in the batch.

  Args:
    emb: a sequence embedding Tensor with shape [batch, max_time, 1, depth].
  Returns:
    a Tensor with shape [batch].
  """
  return tf.cast(tf.reduce_sum(mask_from_embedding(emb), [1, 2, 3]), tf.int32)