MNLI metrics overwritten 
## Description
When running the https://github.com/huggingface/transformers/blob/main/examples/pytorch/text-classification/run_glue.py example for the MNLI dataset, eval metrics calculated for the `validation_mismatched` dataset are overwriting the previously calculated metrics for the standard validation dataset.

## To reproduce
Simply run the default example script for text-classification with `TASK_NAME=mnli`.

## Expected behavior
When evaluation starts, the first one is done on the standard validation MNLI dataset and the metrics are saved to `eval_results.json` and `all_results.json` as:
```json
{
    "epoch": 3.0,
    "eval_accuracy": 0.8501830756712775,
    "eval_loss": 0.4692825376987457,
    "eval_runtime": 15.8047,
    "eval_samples": 9832,
    "eval_samples_per_second": 622.092,
    "eval_steps_per_second": 77.761
}
```
After that, evaluation with `validation_mismatched` happens and unfortunately it's using the same keys as the previous one (`eval_accuracy`, `eval_loss`...), which causes overwriting.
In the ideal case, it would be useful to have both metrics written in these json files.

@sgugger, @patil-suraj