Might be a bug in `AgentPPO`

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.