Gemma finetuning guide is out of date

[Mittagskogel]

The fine-tuning guide https://github.com/huggingface/blog/blob/main/gemma.md#fine-tuning-with-trl for Gemma no longer works with the newest version of TRL. Users should clone https://github.com/huggingface/trl/commit/9bc478ecbb2e9d0c9311784428347c382c05303d.