Metrics calculate error: can only calculate the mean of floating types. Got Bool instead
## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->
     
- `transformers` version: 3.3.1
- Platform: Ubuntu 18.04
- Python version: 3.7.7
- PyTorch version (GPU?): 1.6.0 GPU
- Tensorflow version (GPU?): -
- Using GPU in script?: Yes
- Using distributed or parallel set-up in script?: Yes

### Who can help
Someone working on the examples/GLUE benchmarks 

## Information

Model I am using (Bert, XLNet ...): BERT

The problem arises when using:
* [X] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: (GLUE
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Run QNLI experiment

Error:
```bash
File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/transformers/data/metrics/__init__.py", line 73, in glue_compute_metrics
    return {"acc": simple_accuracy(preds, labels)}
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/transformers/data/metrics/__init__.py", line 34, in simple_accuracy
    return (preds == labels).mean()
RuntimeError: Can only calculate the mean of floating types. Got Bool instead.
Exception ignored in: <function tqdm.__del__ at 0x7f8d1d09ff28>
Traceback (most recent call last):
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/tqdm/std.py", line 1086, in __del__
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/tqdm/std.py", line 1293, in close
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/tqdm/std.py", line 1471, in display
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/tqdm/std.py", line 1089, in __repr__
  File "/home/lucadiliello/anaconda3/envs/nlp/lib/python3.7/site-packages/tqdm/std.py", line 1433, in format_dict
TypeError: cannot unpack non-iterable NoneType object
```

A simple fix is to modify `transformers/data/metrics/__init__.py", line 34, in simple_accuracy` to:
```python
return (preds == labels).to(dtype=torch.float32).mean()
```

## Expected behavior

Metric should be computed without problems
Great, this looks like the correct fix! Do you want to open a PR?<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
