fix squad metric format
One thing for SQuAD is that I wanted to be able to use the SQuAD dataset directly in the metrics and I'm not sure it will be possible with this format.

(maybe it's not really possible in general though) This is kinda related to one thing I had in mind which is that we may want to be able to dump our model predictions in a `Dataset` as well so that we don't keep them in memory (and we can export them in a nice format later as well when we will have a serialization formats).

Maybe this is overkill though, I haven't fully wraped my head around this. I'm also perfectly fine with merging this PR in the current state and working on a larger scope later. This is the format needed to run the official script directly. The format of the squad dataset is different from the input of the metric. 

> One thing for SQuAD is that I wanted to be able to use the SQuAD dataset directly in the metrics and I'm not sure it will be possible with this format.
> 
> (maybe it's not really possible in general though)

Ok I see. I'll try to use the same format Ok with this update I changed the format to fit the squad dataset format.
Now you can do:
```python
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"])
```
The format of the squad metric was wrong.
This should fix #143 

I tested with
```python3
predictions = [
    {'id': '56be4db0acb8001400a502ec', 'prediction_text': 'Denver Broncos'}
]
references = [
    {'answers': [{'text': 'Denver Broncos'}], 'id': '56be4db0acb8001400a502ec'}
]
```