docs: def _embedding_dim(vocab_size):
    """Calculate a reasonable embedding size for a vocabulary.
  
    Rule of thumb is 6 * 4th root of vocab_size.
  
    Args:
      vocab_size: Size of the input vocabulary.
    Returns:
      The embedding size to use.
    Raises:
      ValueError: if `vocab_size` is invalid.
    """
    if not vocab_size or (vocab_size <= 0):
      raise ValueError("Invalid vocab_size %g." % vocab_size)
    return int(round(6.0 * math.sqrt(math.sqrt(vocab_size))))