[recipe] fix: spin fsdp_workers.py bugs (#3544)

Fix TypeError: cannot unpack non-iterable NoneType object

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recipe/spin/fsdp_workers.py

@@ -128,9 +128,7 @@ class SPINRolloutRefWorker(ActorRolloutRefWorker):
             )
 
         if self._is_rollout:
-            self.rollout, self.rollout_sharding_manager = self._build_rollout(
-                trust_remote_code=self.config.model.get("trust_remote_code", False)
-            )
+            self._build_rollout(trust_remote_code=self.config.model.get("trust_remote_code", False))
 
         if self._is_ref:
             self.ref_module_fsdp = self._build_model_optimizer(