Unused function in squad metrics
In the squad metrics file, I noticed an unused function `find_all_best_thresh_v2`

https://github.com/huggingface/transformers/blob/master/src/transformers/data/metrics/squad_metrics.py#L167

It looks to be pertaining to the squad v2.0 dataset type, which has 'impossible' questions for contexts. 

It looks like it was meant to be used something like

```
    if no_answer_probs:
        if version_2_with_negative:
            find_all_best_thresh_v2(evaluation, preds, exact, f1, no_answer_probs, qas_id_to_has_answer)
        else:
             find_all_best_thresh(evaluation, preds, exact, f1, no_answer_probs, qas_id_to_has_answer)
```

here

https://github.com/huggingface/transformers/blob/master/src/transformers/data/metrics/squad_metrics.py#L236
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