text: Title: ValueError: Unknown split "validation". Should be one of ['train'].
  Body: I only run: 
  python train.py --config config/train_cord.yaml --pretrained_model_name_or_path "naver-clova-ix/donut-base" --dataset_name_or_paths '["naver-clova-ix/cord-v2"]' --exp_version "test_experiment"
  It has such an error, why? Thanks for your help!
  
  Traceback (most recent call last):
    File "train.py", line 150, in <module>
      train(config)
    File "train.py", line 87, in train
      sort_json_key=config.sort_json_key,
    File "/home/donut/donut/util.py", line 64, in __init__
      self.dataset = load_dataset(dataset_name_or_path, split=self.split)
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/load.py", line 1644, in load_dataset
      ds = builder_instance.as_dataset(split=split, ignore_verifications=ignore_verifications, in_memory=keep_in_memory)
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/builder.py", line 793, in as_dataset
      disable_tqdm=False,
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/utils/py_utils.py", line 206, in map_nested
      return function(data_struct)
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/builder.py", line 817, in _build_single_dataset
      in_memory=in_memory,
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/builder.py", line 889, in _as_dataset
      in_memory=in_memory,
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 213, in read
      files = self.get_file_instructions(name, instructions, split_infos)
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 187, in get_file_instructions
      name, split_infos, instruction, filetype_suffix=self._filetype_suffix
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 110, in make_file_instructions
      absolute_instructions = instruction.to_absolute(name2len)
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 618, in to_absolute
      return [_rel_to_abs_instr(rel_instr, name2len) for rel_instr in self._relative_instructions]
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 618, in <listcomp>
      return [_rel_to_abs_instr(rel_instr, name2len) for rel_instr in self._relative_instructions]
    File "/home/anaconda3/envs/pytorch/lib/python3.6/site-packages/datasets/arrow_reader.py", line 433, in _rel_to_abs_instr
      raise ValueError('Unknown split "{}". Should be one of {}.'.format(split, list(name2len)))
  ValueError: Unknown split "validation". Should be one of ['train'].