ProbabilityOfImprovement is a BoTorch acquisition-module class for scoring how likely each candidate is to improve on the incumbent, according to a Gaussian-process belief. It favors high predicted means while retaining some interest in uncertain regions. section 4.2.2
BoTorch usage
Create the policy with a GP model and incumbent value, then evaluate a candidate grid: `policy = ProbabilityOfImprovement(model, best_f=train_y.max())`; `scores = policy(xs.unsqueeze(1))`. Optimize those scores with `optimize_acqf`; for the illustrated search, the next point is approximately 0.5985.
