[DRAFT] Refactor DPO

[qgallouedec]

• Deprecate FDivergenceType and FDivergenceConstants
• Deprecate ref_model_init_kwargs -> using a different ref model should be considered as advanced usage, user, will need to load and pass the model themselves
• Add dropout disabling
• Deprecate use_logits_to_keep
• Deprecate label_pad_token_id
• Add truncation_mode