Motivation
MaskablePPO is great for large discrete action space that has many invalid actions at each step, while RecurrentPPO is useful for the agent to has a memory of previous observations and actions taken, which improves it's decision making. Right now, we have to choose between those 2 algorithms and cannot have features of both of them, which would greatly improve agents training when both action masking and sequence processing is helpful.

Feature
MaskableRecurrentPPO - An algorithm that is a combination of MaskablePPO and RecurrentPPO. Or action masking integration to PPO and RecurrentPPO.