[Feature] Automatic LoRA rank recommendation based on dataset size

Description

Choosing the right LoRA rank is largely trial-and-error. An automatic recommendation based on dataset size, task type, and base model would save time.

Use Case

• New users don't know what rank to choose
• Different tasks benefit from different ranks (classification vs generation)
• Could be a --auto-rank flag or a preprocessing analysis step

Based on our experience, rank 16 works for most tasks under 10K samples, but this heuristic could be formalized.