[BUG] Wrong Scores for many SQUAD models
@julien-c
@VictorSanh

## Information

All models trained with run_squad.py have abstaining threshold of 0.0 and possibly wrong evaluation scores.
Those model have their wrong score in their model cards many people rely on. 

For example: ahotrod/electra_large_discriminator_squad2_512

The results with current evaluation script:

```
"exact": 87.09677419354838,
  "f1": 89.98343832723452,
  "total": 11873,
  "HasAns_exact": 84.66599190283401,
  "HasAns_f1": 90.44759839056285,
  "HasAns_total": 5928,
  "NoAns_exact": 89.52060555088309,
  "NoAns_f1": 89.52060555088309,
  "NoAns_total": 5945,
  "best_exact": 87.09677419354838,
  "best_exact_thresh": 0.0,
  "best_f1": 89.98343832723432,
  "best_f1_thresh": 0.0
```

The problem and its fix can be found in: [[Bug Fix] Fix run_squad.py evaluation code doesn't use probabilities](https://github.com/huggingface/transformers/pull/7319) #7319
The problem arises when using:
* [ ] run_squad.py

The tasks I am working on is:
* [ ] SQuAD

## To reproduce

Steps to reproduce the behaviour:

1. run `run_squad.py`

## Expected behaviour:
Resulting 'best_f1_thresh' wont be 0.0 .

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