construct a DistributedStrategy instance if the passed one is None (#21545) (#21567)

Author: lilong12

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

@@ -151,8 +151,10 @@ 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: