[WIP] Iterative training scripts for SPIN and SPPO

[jkx19]

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This is a PR related to #2958 . Specifically, this PR

• Add scripts that describe the iterative training procedure of Self-Play Fine-tuning (https://arxiv.org/abs/2401.01335) and Self-Play Preference Optimization (https://arxiv.org/abs/2405.00675).

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