What is an issue?

Optuna has various pruners, and their handling of nan in intermediate values is different from each other. Checking the source code and documenting each handling would be beneficial for users.

• optuna.pruners.MedianPruner
• optuna.pruners.NopPruner
• optuna.pruners.PatientPruner
• optuna.pruners.PercentilePruner
• optuna.pruners.SuccessiveHalvingPruner
• optuna.pruners.HyperbandPruner
• optuna.pruners.ThresholdPruner