[rllib] Replace ray.get() with ray_get_and_free() to optimize memory usage (#4586)

python/ray/rllib/optimizers/sync_samples_optimizer.py

@@ -10,6 +10,7 @@ from ray.rllib.evaluation.sample_batch import SampleBatch
 from ray.rllib.utils.annotations import override
 from ray.rllib.utils.filter import RunningStat
 from ray.rllib.utils.timer import TimerStat
+from ray.rllib.utils.memory import ray_get_and_free
 
 logger = logging.getLogger(__name__)
 
@@ -50,7 +51,7 @@ class SyncSamplesOptimizer(PolicyOptimizer):
             while sum(s.count for s in samples) < self.train_batch_size:
                 if self.remote_evaluators:
                     samples.extend(
-                        ray.get([
+                        ray_get_and_free([
                             e.sample.remote() for e in self.remote_evaluators
                         ]))
                 else: