examples: add dataset stats logging for DPO preprocessing

[rohansachinpatil]

Summary

Adds dataset statistics logging to the DPO example pipeline to surface
how many samples are dropped during preprocessing.

Details

• Wraps the dataset pipeline with kd.data.WithStats
• Improves observability without changing model or training behavior
• Addresses the existing TODO regarding dropped example visibility

No functional or behavioral changes.

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[rohansachinpatil]

CLA signed.