    lam: if not null, used as the lambda value of generalized advantage estimation (GAE) introduced in "High-Dimensional Continuous Control Using Generalized Advantage Estimation https://arxiv.org/abs/1506.02438. This lambda controls the bias variance tradeoff for GAE. Floating point value between 0 and 1. Lower values correspond to more bias, less variance. Higher values to more variance, less bias. Algorithm becomes A2C(GAE).
