Incorrect `margin` calculation in DPO cookbook

[abheesht17]

We don't need the dpo_beta term while computing the margin here: https://github.com/thinking-machines-lab/tinker-cookbook/blob/main/tinker_cookbook/preference/train_dpo.py#L142.

It's extraneous, since we are already multiplying it in the two lines above: https://github.com/thinking-machines-lab/tinker-cookbook/blob/main/tinker_cookbook/preference/train_dpo.py#L140-L141

[rafailovtm]

Good, point will fix this. Shouldn't affect training right now, it's just for logging purposes.