Better error message when loading adapter models with peft dependency missing

[maxjeblick]

Feature request

Loading adapter models (such as https://huggingface.co/lightonai/MonoQwen2-VL-v0.1/tree/main) fails with an error message when peft isn't installed. The error message
OSError: lightonai/MonoQwen2-VL-v0.1 does not appear to have a file named pytorch_model.bin, model.safetensors, tf_model.h5, model.ckpt or flax_model.msgpack.
is a bit cryptic and requires the user to understand that

• the model that will be loaded is a peft adapter
• peft isn't installed in the current env

To improve UX, it would be useful to show a different error message such as "The model lightonai/MonoQwen2-VL-v0.1 is an adapter model. To load it, you need to install peft (hint: run pip install peft)".

Motivation

Improve UX. The user may get the impression that the model repository is corrupted.

Your contribution

This feature should probably be implemented by core maintainers that are familiar with the internals of the model loading code.