Remove yaml config files

### 🚀 The feature, motivation, and pitch

Was there a reason for using them, or just a legacy feature of TRL?

Advantages of using the Python data class directly are:
- Type safety (it's very easy to make typos in yaml, that Python can catch)
- One less language (most people know yaml to some degree but not well, some won't have used it)
- Code hints (e.g. in VS Code) -> better dev experience

This can be non-breaking (i.e. just change it for the defaults & also the examples, but let users keep parsing their yml files if they already exist). Also seems like a good first issue for someone who wants to start contributing.

### Alternatives

_No response_

### Additional context

_No response_