code: def pad_for_tpu(shapes_dict, hparams, max_length):
    
    padded_shapes = {}
  
    def get_filler(specified_max_length):
      if not specified_max_length:
        return max_length
      return min(specified_max_length, max_length)
  
    inputs_none_filler = get_filler(hparams.max_input_seq_length)
    targets_none_filler = get_filler(hparams.max_target_seq_length)
  
    def pad_one_shape(shape, none_filler):
      return [
          (dim if dim is not None else none_filler) for dim in shape.as_list()
      ]
  
    for key, shape in six.iteritems(shapes_dict):
      if key == "inputs":
        padded_shapes[key] = pad_one_shape(shape, inputs_none_filler)
      elif key == "targets":
        padded_shapes[key] = pad_one_shape(shape, targets_none_filler)
      else:
        padded_shapes[key] = pad_one_shape(shape, max_length)
    return padded_shapes
docs: Pads unknown features' dimensions for TPU.