qlora finetuning script added

[shirinyamani]

In this PR, I tried to add a complete guide on mistral-7b finetuning along with covering the potential questions one could have about Lora implementation. Hope this helps the community!
Cheers

[NazimHAli]

The notebook in this PR is using transformers, peft, bitsandbyters to finetune, not mistral-finetune. There are similar notebooks for these libraries like this with steps to finetune that you can update with some of your changes.