grpo_demo.ipynb: Excessive output

This isn't an outright bug, just something that makes it difficult to work with `grpo_demo.ipynb`.

The cell with `grpo_trainer.train` produces a huge amount of text output. It becomes 95% of the notebook. This makes it difficult to scroll around the notebook or even tell when it's finished. I understand that you would like to show samples of outputs, but if it's important, I'd suggest showing a much smaller selection of outputs to keep the notebook workable.

If you agree with this I'll be happy to help.