DPO Recipe Doc

[salmanmohammadi]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2091

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As of commit 95fb989 with merge base b5d2e63 ():
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[joecummings]

Couple comments, but looks good.