Fix dygraph exit issue (#24347) (#24838)

* add clean up; test=develop

* add import; test=develop

Author: phlrain

python/paddle/fluid/dygraph/base.py

@@ -19,6 +19,7 @@
 import numpy as np
 from paddle.fluid import core
 from paddle.fluid import framework
+from paddle.fluid.multiprocess_utils import CleanupFuncRegistrar
 from .tracer import Tracer
 import logging
 import objgraph
@@ -182,6 +183,9 @@ def enable_dygraph(place=None):
         _functional_dygraph_context_manager = guard(place=place)
         _functional_dygraph_context_manager.__enter__()
 
+        # call disable_dygraph when Python exit
+        CleanupFuncRegistrar.register(disable_dygraph)
+
 
 def disable_dygraph():
     """