Add model selection or ability to overwrite GPT model for concept examples generation

[IhorNi]

Positive sample generation for a concept is a great idea.

Currently, only gpt-3.5 is supported
https://github.com/lilacai/lilac/blob/68e1fe7113a4046a2c741732387bda7cd740c323/lilac/router_concept.py#L273C15-L287

I think it would be great to support other models, at least gpt-4.

Thanks!

[nsthorat]

+1, we're gonna prioritize this!