Dpo penalty update

[Eugene-hu]

• Adds additional checks for empty strings and smaller completions
• Adds a penalty for repeated tokens akin to huggingface's repeat penalty during generation (https://github.com/huggingface/transformers/blob/v4.32.0/src/transformers/generation/logits_process.py#L279)
• The penalty will deter repeated tokens with subsequently heavier penalties each time it occurs.