Function model_cache_path 

pub fn model_cache_path(model_id: &str) -> Option<PathBuf>

Return the cache sub-directory path for a given model ID.

E.g. for "bartowski/Qwen2.5-1.5B-Instruct-GGUF" the cache directory is <hf_cache>/models--bartowski--Qwen2.5-1.5B-Instruct-GGUF.