Fix squad V2 metric script
The current squad v2 metric doesn't work with the squad (v1 or v2) datasets. The script is copied from `squad_evaluate` in transformers that requires the labels (with multiple answers) to be like this:
```
references = [{'id': 'a', 'answers': [
    {'text': 'Denver Broncos', 'answer_start': 177},
    {'text': 'Denver Broncos', 'answer_start': 177}
]}]
```
while the dataset had references like this:
```
references = [{'id': 'a', 'answers': 
    {'text': ['Denver Broncos' 'Denver Broncos'], 'answer_start': [177, 177]}
}]
```

Using one or the other format fails with the current squad v2 metric:
```
from datasets import load_metric
metric = load_metric("squad_v2")
predictions = [{'id': 'a', 'prediction_text': 'Denver Broncos', 'no_answer_probability': 0.0}]
references = [{'id': 'a', 'answers': [
    {'text': 'Denver Broncos', 'answer_start': 177},
    {'text': 'Denver Broncos', 'answer_start': 177}
]}]
metric.compute(predictions=predictions, references=references)
```
fails as well as
```
from datasets import load_metric
metric = load_metric("squad_v2")
predictions = [{'id': 'a', 'prediction_text': 'Denver Broncos', 'no_answer_probability': 0.0}]
references = [{'id': 'a', 'answers': 
    {'text': ['Denver Broncos' 'Denver Broncos'], 'answer_start': [177, 177]}
}]
metric.compute(predictions=predictions, references=references)
```

This is because arrow reformats the references behind the scene.

With this PR (tested locally), both the snippets up there work and return proper results.
The script with changes is used and tested in [#8924](https://github.com/huggingface/transformers/pull/8924). It gives the same results as the old `evaluate_squad` function when used on the same predictions. merging since the CI is fixed on master