can not flatten natural_questions dataset
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same issue. downgrade it to a lower version. Thanks for reporting, I'll take a look tomorrow :)
## Describe the bug
after downloading the natural_questions dataset, can not flatten the dataset considering there are `long answer` and `short answer` in `annotations`.

## Steps to reproduce the bug
```python
from datasets import load_dataset

dataset = load_dataset('natural_questions',cache_dir = 'data/dataset_cache_dir')
dataset['train'].flatten()
```

## Expected results
a dataset with `long_answer` as features

## Actual results
Traceback (most recent call last):
  File "temp.py", line 5, in <module>
    dataset['train'].flatten()
  File "/Users/hannibal046/anaconda3/lib/python3.8/site-packages/datasets/fingerprint.py", line 413, in wrapper
    out = func(self, *args, **kwargs)
  File "/Users/hannibal046/anaconda3/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 1296, in flatten
    dataset._data = update_metadata_with_features(dataset._data, dataset.features)
  File "/Users/hannibal046/anaconda3/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 536, in update_metadata_with_features
    features = Features({col_name: features[col_name] for col_name in table.column_names})
  File "/Users/hannibal046/anaconda3/lib/python3.8/site-packages/datasets/arrow_dataset.py", line 536, in <dictcomp>
    features = Features({col_name: features[col_name] for col_name in table.column_names})
KeyError: 'annotations.long_answer'
## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 1.8.13
- Platform: MBP
- Python version: 3.8
- PyArrow version: 6.0.1

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