fix conflicts to beautify the code

colossalai/zero/gemini/placement_policy.py

@@ -20,7 +20,7 @@ class PlacementPolicy(ABC):
 
     def __init__(
         self,
-        gemini_manager: "GeminiManager",
+        gemini_manager: "GeminiManager",  # TODO @botbw: solve circular import
         chunk_manager: ChunkManager,
         mem_stats_collector: Optional[ChunkMemStatsCollector] = None,
         max_prefetch: int = 0,
@@ -41,9 +41,8 @@ class PlacementPolicy(ABC):
     ) -> None:
         raise NotImplementedError
 
-    @abstractmethod
     def get_prefetch_chunks(self) -> List[Chunk]:
-        raise NotImplementedError
+        return []  # no prefetch by default
 
 
 class StaticPlacementPolicy(PlacementPolicy):