[BugFix] Fix wrong assertion about collector and buffer (#3176)

Author: vmoens

torchrl/trainers/algorithms/configs/trainers.py

@@ -101,7 +101,9 @@ def _make_sac_trainer(*args, **kwargs) -> SACTrainer:
         elif replay_buffer is not None:
             collector = collector(replay_buffer=replay_buffer)
     elif getattr(collector, "replay_buffer", None) is None:
-        if collector.replay_buffer is None or replay_buffer is None:
+        if async_collection and (
+            collector.replay_buffer is None or replay_buffer is None
+        ):
             raise ValueError(
                 "replay_buffer must be provided when async_collection is True"
             )
@@ -230,7 +232,7 @@ def _make_ppo_trainer(*args, **kwargs) -> PPOTrainer:
             collector = collector()
         else:
             collector = collector(replay_buffer=replay_buffer)
-    elif getattr(collector, "replay_buffer", None) is None:
+    elif async_collection and getattr(collector, "replay_buffer", None) is None:
         raise RuntimeError(
             "replay_buffer must be provided when async_collection is True"
         )