Extract a base class from A2CTrainer.
Extract a base class from A2CTrainer.

This way PPO and AWR don't need to inherit from A2CTrainer, which was confusing because A2C is a concrete algorithm, so shouldn't be a base class. Also simplified the definitions of RL loss functions a bit by passing log probabilities directly to them.

PiperOrigin-RevId: 304022981

google/trax,google/trax