refactor: reorganize dataset into lm/vlm/vam subdirectories

rename minimind→omni in model_type, eval/convert scripts, and docs

reorganize configs into subdirs lm/vlm/vam, remove runs/, add interview docs for pretrain & full_sft

cleanup: remove deprecated doc files, update src training utils

Add local training logger that writes to {save_dir}/{save_weight}.log

Add tokenizer training script and save trained tokenizer into checkpoint dir

Flatten src layout: drop omni namespace, modules live directly under src/