Fix AttributeError in diffusion_dpo ipo loss path

The IPO branch in `train_diffusion_dpo.py` references `args.beta`, but
this script only defines `--beta_dpo` — there is no `--beta` argument.
Running training with `--loss_type ipo` crashes immediately at the loss
computation with:

    AttributeError: 'Namespace' object has no attribute 'beta'

The `sigmoid` and `hinge` branches right above correctly use
`args.beta_dpo`, so IPO was clearly meant to use the same hyperparameter.
Swap to `args.beta_dpo` to unbreak `--loss_type ipo`.

Author: Ricardo-M-L

examples/research_projects/diffusion_dpo/train_diffusion_dpo.py

@@ -881,7 +881,7 @@ def collate_fn(examples):
                 elif args.loss_type == "hinge":
                     loss = torch.relu(1 - args.beta_dpo * logits).mean()
                 elif args.loss_type == "ipo":
-                    losses = (logits - 1 / (2 * args.beta)) ** 2
+                    losses = (logits - 1 / (2 * args.beta_dpo)) ** 2
                     loss = losses.mean()
                 else:
                     raise ValueError(f"Unknown loss type {args.loss_type}")