set default value to strategy in distributed_optimizer test=develop

python/paddle/distributed/fleet/base/fleet_base.py

@@ -16,6 +16,7 @@ from __future__ import print_function
 import paddle
 from .strategy_compiler import StrategyCompiler
 from .meta_optimizer_factory import MetaOptimizerFactory
+from .distributed_strategy import DistributedStrategy
 from .runtime_factory import RuntimeFactory
 from .util_factory import UtilFactory
 
@@ -209,7 +210,7 @@ class Fleet(object):
         assert self._runtime_handle is not None
         self._runtime_handle._stop_worker()
 
-    def distributed_optimizer(self, optimizer, strategy):
+    def distributed_optimizer(self, optimizer, strategy=None):
         """
         distirbuted_optimizer
         Returns:
@@ -225,6 +226,8 @@ class Fleet(object):
             optimizer = fleet.distributed_optimizer(optimizer, strategy=strategy)
         """
         self.user_defined_optimizer = optimizer
+        if strategy == None:
+            strategy = DistributedStrategy()
         self.user_defined_strategy = strategy
         self.valid_strategy = None
         return self