Add FLOPS to reporting of results

Per @lewtun:

```
Should we also report FLOPs (or similar)? I believe one can get this from the Trainer pretty easily (if not by default): https://huggingface.co/docs/transformers/v4.22.2/en/main_classes/trainer#transformers.Trainer.floating_point_ops

This would allow people to plot performance vs compute, which is arguably more meaningful (modulo caveats around hardware differences)
```