docs: def pad_tensor(tensor, length, padding_index=DEFAULT_PADDING_INDEX):
      """ Pad a ``tensor`` to ``length`` with ``padding_index``.
  
      Args:
          tensor (torch.Tensor [n, ...]): Tensor to pad.
          length (int): Pad the ``tensor`` up to ``length``.
          padding_index (int, optional): Index to pad tensor with.
  
      Returns
          (torch.Tensor [length, ...]) Padded Tensor.
      """
      n_padding = length - tensor.shape[0]
      assert n_padding >= 0
      if n_padding == 0:
          return tensor
      padding = tensor.new(n_padding, *tensor.shape[1:]).fill_(padding_index)
      return torch.cat((tensor, padding), dim=0)