map/filter multiprocessing raises errors and corrupts datasets 
 After upgrading to the 1.0 started seeing errors in my data loading script after enabling multiprocessing.

```python
    ...
    ner_ds_dict = ner_ds.train_test_split(test_size=test_pct, shuffle=True, seed=seed)
    ner_ds_dict["validation"] = ner_ds_dict["test"]
    rel_ds_dict = rel_ds.train_test_split(test_size=test_pct, shuffle=True, seed=seed)
    rel_ds_dict["validation"] = rel_ds_dict["test"]
    return ner_ds_dict, rel_ds_dict
```

The first train_test_split, `ner_ds`/`ner_ds_dict`, returns a `train` and `test` split that are iterable.
The second, `rel_ds`/`rel_ds_dict` in this case, returns a Dataset dict that has rows but if selected from or sliced into into returns an empty dictionary. eg `rel_ds_dict['train'][0] == {}` and `rel_ds_dict['train'][0:100] == {}`.

Ok I think I know the problem -- the rel_ds was mapped though a mapper with `num_proc=12`. If I remove `num_proc`. The dataset loads.

I also see errors with other map and filter functions when `num_proc` is set.

```
Done writing 67 indices in 536 bytes .
Done writing 67 indices in 536 bytes .
Fatal Python error: PyCOND_WAIT(gil_cond) failed
``` 
 @lhoestq  

Thanks for taking a look. I pulled the master but I still see the key error.

```
To disable this warning, you can either:
        - Avoid using `tokenizers` before the fork if possible
        - Explicitly set the environment variable TOKENIZERS_PARALLELISM=(true | false)
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Traceback (most recent call last):                                                                                      | 0/1 [00:00<?, ?ba/s]
  File "text2struct/run_model.py", line 372, in <module>
    main()
  File "text2struct/run_model.py", line 368, in main                                                                    | 0/1 [00:00<?, ?ba/s]
    run_model(auto_envvar_prefix="GFB_CIES")  # pragma: no cover
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)                                                                                   | 0/1 [00:00<?, ?ba/s]
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 1236, in invoke
    return Command.invoke(self, ctx)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "text2struct/run_model.py", line 136, in run_model
    ctx.invoke(ctx.command.commands[config_dict["mode"]])
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/click/decorators.py", line 21, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "text2struct/run_model.py", line 187, in train
    run_train_model(_parse_subcommand(ctx))
  File "text2struct/run_model.py", line 241, in run_train_model
    checkpoint_steps=config.train.checkpoint_steps,
  File "/Users/timothy.laurent/src/inv-text2struct/text2struct/model/train.py", line 153, in alternate_training
    max_len=config.model.dim.max_len,
  File "/Users/timothy.laurent/src/inv-text2struct/text2struct/model/dataset.py", line 466, in load_prodigy_tf_datasets
    folder, file_patterns, vocabulary, cache_dir=cache_dir, test_pct=test_pct
  File "/Users/timothy.laurent/src/inv-text2struct/text2struct/model/dataset.py", line 447, in load_prodigy_arrow_datasets
    fn_kwargs=dict(tokenizer=vocabulary.tokenizer, vocabulary=vocabulary),
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/datasets/arrow_dataset.py", line 1224, in map
    update_data = does_function_return_dict(test_inputs, test_indices)
  File "/Users/timothy.laurent/.virtualenvs/inv-text2struct/lib/python3.6/site-packages/datasets/arrow_dataset.py", line 1195, in does_function_return_dict
    function(*fn_args, indices, **fn_kwargs) if with_indices else function(*fn_args, **fn_kwargs)
  File "/Users/timothy.laurent/src/inv-text2struct/text2struct/model/dataset.py", line 348, in relation_ds_factory
    ner_labels = example["ner_labels"]
KeyError: 'ner_labels'

```