Use `AsRef<Path>` instead of `&str` for file operations

[Colerar]

e.g.

https://github.com/burn-rs/burn/blob/fb2a71bb81e1a688b4cdae38729b24dd9361283f/burn-core/src/config.rs#L47

This allow caller pass PathBuf, String, &str and all types implemented AsRef<Path>, and it's API-compatible, it won't break any existing code.