Title: Cannot load udpos subsets from xtreme dataset using load_dataset()
State: closed
Labels: description: Good for newcomers
name: good first issue

Hello, 

I am trying to load the udpos English subset from xtreme dataset, but this faces an error during loading. I am using datasets v1.4.1, pip install. I have tried with other udpos languages which also fail, though loading a different subset altogether (such as XNLI) has no issue. I have also tried on Colab and faced the same error. 

Reprex is: 

`from datasets import load_dataset `
`dataset = load_dataset('xtreme', 'udpos.English')`

The error is: 
`KeyError: '_'`

The full traceback is: 
KeyError                                  Traceback (most recent call last)
<ipython-input-5-7181359ea09d> in <module>
      1 from datasets import load_dataset
----> 2 dataset = load_dataset('xtreme', 'udpos.English')

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\load.py in load_dataset(path, name, data_dir, data_files, split, cache_dir, features, download_config, download_mode, ignore_verifications, keep_in_memory, save_infos, script_version, use_auth_token, **config_kwargs)
    738 
    739     # Download and prepare data
--> 740     builder_instance.download_and_prepare(
    741         download_config=download_config,
    742         download_mode=download_mode,

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, try_from_hf_gcs, dl_manager, base_path, use_auth_token, **download_and_prepare_kwargs)
    576                             logger.warning("HF google storage unreachable. Downloading and preparing it from source")
    577                     if not downloaded_from_gcs:
--> 578                         self._download_and_prepare(
    579                             dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    580                         )

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    654             try:
    655                 # Prepare split will record examples associated to the split
--> 656                 self._prepare_split(split_generator, **prepare_split_kwargs)
    657             except OSError as e:
    658                 raise OSError(

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\builder.py in _prepare_split(self, split_generator)
    977                 generator, unit=" examples", total=split_info.num_examples, leave=False, disable=not_verbose
    978             ):
--> 979                 example = self.info.features.encode_example(record)
    980                 writer.write(example)
    981         finally:

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in encode_example(self, example)
    946     def encode_example(self, example):
    947         example = cast_to_python_objects(example)
--> 948         return encode_nested_example(self, example)
    949 
    950     def encode_batch(self, batch):

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in encode_nested_example(schema, obj)
    840     # Nested structures: we allow dict, list/tuples, sequences
    841     if isinstance(schema, dict):
--> 842         return {
    843             k: encode_nested_example(sub_schema, sub_obj) for k, (sub_schema, sub_obj) in utils.zip_dict(schema, obj)
    844         }

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in <dictcomp>(.0)
    841     if isinstance(schema, dict):
    842         return {
--> 843             k: encode_nested_example(sub_schema, sub_obj) for k, (sub_schema, sub_obj) in utils.zip_dict(schema, obj)
    844         }
    845     elif isinstance(schema, (list, tuple)):

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in encode_nested_example(schema, obj)
    868     # ClassLabel will convert from string to int, TranslationVariableLanguages does some checks
    869     elif isinstance(schema, (ClassLabel, TranslationVariableLanguages, Value, _ArrayXD)):
--> 870         return schema.encode_example(obj)
    871     # Other object should be directly convertible to a native Arrow type (like Translation and Translation)
    872     return obj

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in encode_example(self, example_data)
    647         # If a string is given, convert to associated integer
    648         if isinstance(example_data, str):
--> 649             example_data = self.str2int(example_data)
    650 
    651         # Allowing -1 to mean no label.

~\Anaconda3\envs\mlenv\lib\site-packages\datasets\features.py in str2int(self, values)
    605                 if value not in self._str2int:
    606                     value = value.strip()
--> 607                 output.append(self._str2int[str(value)])
    608             else:
    609                 # No names provided, try to integerize

KeyError: '_'



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