[skip ci] restore import statement

ml-agents/mlagents/trainers/trainer_util.py

 from mlagents.trainers.ghost.trainer import GhostTrainer
 from mlagents.trainers.ghost.controller import GhostController
 from mlagents.trainers.settings import TrainerSettings, TrainerType
-from mlagents_distributed.trainers.ppo import DistributedPPOTrainer
-from mlagents_distributed.trainers.sac import DistributedSACTrainer
+from mlagents_distributed.trainers.ppo.trainer import DistributedPPOTrainer
+from mlagents_distributed.trainers.sac.trainer import DistributedSACTrainer
 
 logger = get_logger(__name__)