[Speech recognition examples] num_processing_workers not allowed to be set
## Environment info
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- `transformers` version: master
- Platform: ubuntu
- Python version:
- PyTorch version (GPU?): 
- Tensorflow version (GPU?):
- Using GPU in script?:
- Using distributed or parallel set-up in script?:

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## Information

Model I am using (Bert, XLNet ...):

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [x] an official GLUE/SQUaD task: (give the name) commonvoice
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. run one of the official speech recognition examples with num_processing_workers specified
2.
3.

```
Traceback (most recent call last):
  File "/mnt/f/Codes/Python Apps/asr/speech_recognition_seq2seq.py", line 504, in <module>
    main()
  File "/mnt/f/Codes/Python Apps/asr/speech_recognition_seq2seq.py", line 380, in main
    vectorized_datasets = raw_datasets.map(
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/dataset_dict.py", line 494, in map
    {
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/dataset_dict.py", line 495, in <dictcomp>
    k: dataset.map(
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 2142, in map
    shards = [
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 2143, in <listcomp>
    self.shard(num_shards=num_proc, index=rank, contiguous=True, keep_in_memory=keep_in_memory)
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 3168, in shard
    return self.select(
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 485, in wrapper
    out: Union["Dataset", "DatasetDict"] = func(self, *args, **kwargs)
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/fingerprint.py", line 411, in wrapper
    out = func(self, *args, **kwargs)
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 2760, in select
    return self._new_dataset_with_indices(indices_buffer=buf_writer.getvalue(), fingerprint=new_fingerprint)
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 2671, in _new_dataset_with_indices
    return Dataset(
  File "/home/flozi/anaconda3/lib/python3.9/site-packages/datasets/arrow_dataset.py", line 661, in __init__
    raise ValueError(
ValueError: External features info don't match the dataset:
Got
{'client_id': Value(dtype='string', id=None), 'path': Value(dtype='string', id=None), 'audio': Audio(sampling_rate=16000, mono=True, _storage_dtype='string', id=None), 'sentence': Value(dtype='string', id=None), 'up_votes': Value(dtype='int64', id=None), 'down_votes': Value(dtype='int64', id=None), 'age': Value(dtype='string', id=None), 'gender': Value(dtype='string', id=None), 'accent': Value(dtype='string', id=None), 'locale': Value(dtype='string', id=None), 'segment': Value(dtype='string', id=None)}
with type
struct<client_id: string, path: string, audio: string, sentence: string, up_votes: int64, down_votes: int64, age: string, gender: string, accent: string, locale: string, segment: string>

but expected something like
{'client_id': Value(dtype='string', id=None), 'path': Value(dtype='string', id=None), 'audio': {'path': Value(dtype='string', id=None), 'bytes': Value(dtype='binary', id=None)}, 'sentence': Value(dtype='string', id=None), 'up_votes': Value(dtype='int64', id=None), 'down_votes': Value(dtype='int64', id=None), 'age': Value(dtype='string', id=None), 'gender': Value(dtype='string', id=None), 'accent': Value(dtype='string', id=None), 'locale': Value(dtype='string', id=None), 'segment': Value(dtype='string', id=None)}
with type
struct<client_id: string, path: string, audio: struct<path: string, bytes: binary>, sentence: string, up_votes: int64, down_votes: int64, age: string, gender: string, accent: string, locale: string, segment: string>
```

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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

Actually, I've seen this error before as well and it confused me a lot as well - thanks a lot for flagging it here @flozi00! I think it only happens though when the audio has to be resampled (like for Common Voice). 

@lhoestq - it's a weird bug with `datasets` I think. I'll try to get a minimum reproducible code-snippet.<|||||>@lhoestq @albertovilla @mariosasko here is a minimal reproducible bug report:

```python
from datasets import load_dataset, DatasetDict
import datasets
from transformers import AutoFeatureExtractor

raw_datasets = DatasetDict()

raw_datasets["train"] = load_dataset("common_voice", "ab", split="train")

feature_extractor = AutoFeatureExtractor.from_pretrained("facebook/wav2vec2-base")

raw_datasets = raw_datasets.cast_column(
    "audio", datasets.features.Audio(sampling_rate=feature_extractor.sampling_rate)
)

num_workers = 16

def prepare_dataset(batch):
    sample = batch["audio"]

    inputs = feature_extractor(sample["array"], sampling_rate=sample["sampling_rate"])
    batch["input_values"] = inputs.input_values[0]
    batch["input_length"] = len(batch["input_values"])
    return batch

raw_datasets.map(
    prepare_dataset,
    remove_columns=next(iter(raw_datasets.values())).column_names,
    num_proc=16,
    desc="preprocess datasets",
)
```<|||||>Opened an issue on datasets: https://github.com/huggingface/datasets/issues/3497 (feel free to disregard issue here)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Think we can close this one no @lhoestq ?<|||||>Yes indeed, thanks for the heads up !