Add "repeat" feature for fine tuning

Each concept can have a different repeat in DreamBooth method. But metadata .json (fine tuning method) does not have the feature.

One idea is that the folder name for fine tuning can have repeats like ``_``. concept is ignored. If the repeat is not provided, it become 1. And merge_captions or merge_dd_tags script will append the repeat value to json.

Start by reading the merge_captions and merge_dd_tags scripts and trace how fine-tuning metadata is created. Check how folder names are currently parsed, then implement the _ convention with a default repeat of 1 and confirm that the repeat value is appended to the generated JSON.

Written by the indexing model from the issue text.