Update train_dpo.py for calibrated DPO v2 training

Security: remove hardcoded tokens from code/train_dpo.py, use env/arg only

Add SFT auxiliary loss (0.5x) and boost DPO learning rate to 1.5e-5 for 500M model

Upload latest DPO v1 alignment script: code/train_dpo.py

Update code/train_dpo.py with user enhancements

Update train_dpo.py with complete DDP multi-GPU and memory management

Upload native PyTorch DPO training engine