[MNLI example] Prevent overwriting matched with mismatched metrics
# What does this PR do?
Fixes https://github.com/huggingface/transformers/issues/16474.

The fix appends `_mm` to the metrics for `mnli-mm` evaluation dataset to prevent overwriting eval metrics from the `mnli` dataset, and writes them together in `eval_results.json` and `all_results.json`. The MNLI-metrics look like this:

```json
{
    "eval_accuracy": 0.36067244014263883,
    "eval_accuracy_mm": 0.35506509357200977,
    "eval_loss": 1.158889889717102,
    "eval_loss_mm": 1.1670204401016235,
    "eval_runtime": 16.4691,
    "eval_runtime_mm": 15.9496,
    "eval_samples": 9815,
    "eval_samples_mm": 9832,
    "eval_samples_per_second": 595.964,
    "eval_samples_per_second_mm": 616.44,
    "eval_steps_per_second": 18.641,
    "eval_steps_per_second_mm": 19.311
}
```

@sgugger, @patil-suraj
_The documentation is not available anymore as the PR was closed or merged._<|||||>The CI failure is unrelated to this PR (actually investigating it right now), so we can merge safely :-)