bug fix (#23193)

test=develop

python/paddle/fluid/dygraph/base.py

@@ -105,6 +105,7 @@ def enable_dygraph(place=None):
             print(fluid.in_dygraph_mode())  # False
     """
     global _functional_dygraph_context_manager
+    if _functional_dygraph_context_manager is None:
         _functional_dygraph_context_manager = guard(place=place)
         _functional_dygraph_context_manager.__enter__()