We have implemented the REINFORCE++ algorithm.

To use it, specify the parameter
`algorithm.adv_estimator=reinforce_plus_plus`.

Preliminary performance evaluations were conducted within the
[Unakar/Logic-RL](https://github.com/Unakar/Logic-RL) project, a
reproduction of DeepSeek R1 Zero on the 2K Tiny Logic Puzzle Dataset.
Results indicate that our REINFORCE++ implementation exhibits
performance and training stability comparable to, or potentially
exceeding, that of PPO and GRPO.

Related issue: #68