[docs] LoRA for token classification

[stevhliu]

Moves the LoRA for token classification notebook into the docs and adapts it for a less-complicated example instead of training LayoutLM on annotated forms.

• complete the training section with either a native PyTorch loop or Trainer

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[pacman100]

Thanks a lot @stevhliu for simplifying the token classification example to an interesting bionlp dataset!

Left few comments

[MKhalusova]

Nice guide! I left a few nits here and there.

[pacman100]

Thank you @stevhliu for iterating, LGTM! 🚀
I really like the example and the dataset used.