[Rust] GrpoRewards: Implement Policy Evaluation Functions

[kyjohnso]

@nikg4 @taenin what other functions should we put into the GrpoRewards struct? other rewards functions? Here is a first cut from sonnet

2. Policy Evaluation Functions:
   • evaluate_policy(policy, environment) - Measure performance of a given policy
   • compute_advantage(values, rewards) - Calculate advantage estimates
   • compute_returns(rewards, gamma) - Calculate cumulative returns

Implement evaluate_policy, compute_advantage, compute_returns functions in rust

Originally posted by @kyjohnso in #13