Might be a bug in AgentPPO

[Die-Xie]

In AgnetPPO, act_target and cri_target may not be properly initialized and are only set to None in the parent class, which could lead to errors when calling save_or_load().

The same issue may also occur in agents such as AgentDiscretePPO, AgentDiscreteA2C, and other similar agents.

I encountered this problem while training with AgentPPO in a custom environment. It was resolved by initializing act_target and cri_target with self.act and self.cri , respectively, during the initialization of AgentPPO.