`compute_metrics` in `GRPOTrainer`

### Feature request

Can you please add `compute_metrics` in the `GRPOTrainer`?

I can see its available in other, i.e., [DPOTrainer](https://huggingface.co/docs/trl/v0.15.2/dpo_trainer#trl.DPOTrainer.compute_metrics)

### Motivation

1. We can compute accuracy or downstream eval metrics over the evaluation dataset
2. That will help in more fine-grained logging
3. Better metric for `metric_for_best_model`

### Your contribution

I am open to submit a PR