bugfix: construct a DistributedStrategy instance if the passed one is None (#21545)

python/paddle/fluid/incubate/fleet/collective/__init__.py

@@ -191,8 +191,10 @@ class CollectiveOptimizer(DistributedOptimizer):
     """
 
     def __init__(self, optimizer, strategy=DistributedStrategy()):
+        if strategy is None:
+            strategy = DistributedStrategy()
         super(CollectiveOptimizer, self).__init__(optimizer, strategy)
-        if strategy is not None and strategy.forward_recompute:
+        if strategy.forward_recompute:
             self.forward_recompute = True
             self.recompute_checkpoints = strategy.recompute_checkpoints
         else: