ArrowTypeError in squad metrics
{'login': 'patil-suraj', 'id': 27137566, 'node_id': 'MDQ6VXNlcjI3MTM3NTY2', 'avatar_url': 'https://avatars.githubusercontent.com/u/27137566?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/patil-suraj', 'html_url': 'https://github.com/patil-suraj', 'followers_url': 'https://api.github.com/users/patil-suraj/followers', 'following_url': 'https://api.github.com/users/patil-suraj/following{/other_user}', 'gists_url': 'https://api.github.com/users/patil-suraj/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/patil-suraj/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/patil-suraj/subscriptions', 'organizations_url': 'https://api.github.com/users/patil-suraj/orgs', 'repos_url': 'https://api.github.com/users/patil-suraj/repos', 'events_url': 'https://api.github.com/users/patil-suraj/events{/privacy}', 'received_events_url': 'https://api.github.com/users/patil-suraj/received_events', 'type': 'User', 'site_admin': False}
[{'id': 2067393914, 'node_id': 'MDU6TGFiZWwyMDY3MzkzOTE0', 'url': 'https://api.github.com/repos/huggingface/datasets/labels/metric%20bug', 'name': 'metric bug', 'color': '25b21e', 'default': False, 'description': 'A bug in a metric script'}]
[]
There was an issue in the format, thanks.
Now you can do
```python3
squad_dset = nlp.load_dataset("squad")
squad_metric = nlp.load_metric("/Users/quentinlhoest/Desktop/hf/nlp-bis/metrics/squad")
predictions = [
    {"id": v["id"], "prediction_text": v["answers"]["text"][0]}  # take first possible answer
    for v in squad_dset["validation"]
]
squad_metric.compute(predictions, squad_dset["validation"])
```

and the expected format is 
```
Args:
    predictions: List of question-answers dictionaries with the following key-values:
        - 'id': id of the question-answer pair as given in the references (see below)
        - 'prediction_text': the text of the answer
    references: List of question-answers dictionaries with the following key-values:
        - 'id': id of the question-answer pair (see above),
        - 'answers': a Dict {'text': list of possible texts for the answer, as a list of strings}
```
`squad_metric.compute` is giving following error
```
ArrowTypeError: Could not convert [{'text': 'Denver Broncos'}, {'text': 'Denver Broncos'}, {'text': 'Denver Broncos'}] with type list: was not a dict, tuple, or recognized null value for conversion to struct type
```

This is how my predictions and references look like
```
predictions[0]
# {'id': '56be4db0acb8001400a502ec', 'prediction_text': 'Denver Broncos'}
```

```
references[0]
# {'answers': [{'text': 'Denver Broncos'},
  {'text': 'Denver Broncos'},
  {'text': 'Denver Broncos'}],
 'id': '56be4db0acb8001400a502ec'}
```

These are structured as per the `squad_metric.compute` help string.
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