[ABL] Clean Up HF Conversion & Adapter

[flxst]

Modalities currently supports two separate ways to convert Modalities checkpoints such that they can be used with HuggingFace:

1. Conversion to an HF Adapter
2. Conversion to an HF Checkpoint

Recent PRs include e.g. #285, #306 for 1) and #305 for 2)

There are a few issues that need to be resolved:

• We need to decide whether we want to continue supporting both 1) and 2). Maybe 1) is enough?
• The term "conversion" is used for both 1) and 2). We should perhaps rename everything regarding 2)
• Regarding 2) - if we decide to keep it - some refactoring is needed. The directories in src/modalities/models should be moved elsewhere. There is also code in src/modalities/checkpointing and a modalities CLI endpoint.
• Regarding 1), we should implement a modalities CLI endpoint.