Fix AttributeError in diffusion_dpo --loss_type ipo

[Ricardo-M-L]

What does this PR do?

Fixes an AttributeError in examples/research_projects/diffusion_dpo/train_diffusion_dpo.py that crashes the IPO loss path.

The IPO branch references args.beta:

elif args.loss_type == "ipo":
    losses = (logits - 1 / (2 * args.beta)) ** 2
    loss = losses.mean()

but this script only exposes --beta_dpo:

parser.add_argument(
    \"--beta_dpo\",
    type=int,
    default=2500,
    help=\"DPO KL Divergence penalty.\",
)

So running with --loss_type ipo crashes immediately with:

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

The sigmoid and hinge branches above correctly use args.beta_dpo, and the SDXL sibling train_diffusion_dpo_sdxl.py also only uses args.beta_dpo. This was clearly intended to use the same hyperparameter.

Fix

 elif args.loss_type == \"ipo\":
-    losses = (logits - 1 / (2 * args.beta)) ** 2
+    losses = (logits - 1 / (2 * args.beta_dpo)) ** 2
     loss = losses.mean()

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Who can review?

@sayakpaul