What exactly would you like to know?
Currently, we are only supporting PPO and the LearningAgentsTrainer is pretty closely tied to PPO. The PPO is our own implementation with some features we think are beneficial to game devs.
You can find the PPO code in:
{Your-Workspace}\Engine\Plugins\Experimental\LearningAgents\Content\Python\train_ppo.py and the related files.