Fix async training error in last rollout (THUDM#452)

Author: fzyzcjy

train_async.py

@@ -53,7 +53,7 @@ def train(args):
 
         if (rollout_id + 1) % args.update_weights_interval == 0:
             # sync generate before update weights to prevent update weight in the middle of generation
-            rollout_data_curr_ref = ray.get(rollout_data_next_future)
+            rollout_data_curr_ref = ray.get(x) if (x := rollout_data_next_future) is not None else None
             rollout_data_next_future = None
             actor_model.update_weights()