Refactor main fine-tuning script

**Is your feature request related to a problem? Please describe.**
The main fine-tuning script `keys_values/finetune/longcontext_full.py` has lots of code not really used anymore.

**Describe the solution you'd like**
Remove code or move it to functions. Make sure the script is easy to understand and modify.

Start by reading keys_values/finetune/longcontext_full.py and trace which parts of the fine-tuning script are still used. Refactor or remove obsolete code while keeping the script easy to understand and modify; done means the resulting script has no unused code identified during the review.

Written by the indexing model from the issue text.