Gemma-peft tutorial does not perform expected fine-tuning

Executing the script described in the tutorial https://github.com/huggingface/blog/blob/main/gemma-peft.md throws this warning:

> You passed a dataset that is already processed (contains an `input_ids` field) together with a formatting function. Therefore `formatting_func` will be ignored. Either remove the `formatting_func` or pass a dataset that is not already processed.

As `formatting_func` is ignored, the model is not correctly fine-tuned. The script works but there is no guarantee that the model ouput the format `Quote: [...] Author: [...]`.

Proposed solution in this PR: https://github.com/huggingface/blog/pull/3260

Read gemma-peft.md and run the tutorial script to reproduce the warning about the ignored formatting_func. Review proposed pull request #3260 for the intended change. Done means the formatting function is applied and the fine-tuned model is expected to output the Quote and Author format.

Written by the indexing model from the issue text.