Make the distillation process not save teacher variables in PaddleSlim (#19633)

* split teacher checkpoints with student checkpoints, test=develop

* add unittest for graph.merge(), test=develop

python/paddle/fluid/contrib/slim/graph/graph_wrapper.py

@@ -205,6 +205,7 @@ class GraphWrapper(object):
         super(GraphWrapper, self).__init__()
         self.program = Program() if program is None else program
         self.persistables = {}
+        self.teacher_persistables = {}
         for var in self.program.list_vars():
             if var.persistable:
                 self.persistables[var.name] = var
@@ -306,6 +307,8 @@ class GraphWrapper(object):
             graph(GraphWrapper): The graph to be merged by current graph.
         """
         for var in graph.program.list_vars():
+            if var.persistable:
+                self.teacher_persistables[var.name] = var
             new_var = self.program.global_block()._clone_variable(
                 var, force_persistable=False)
             new_var.stop_gradient = var.stop_gradient
@@ -479,7 +482,7 @@ class GraphWrapper(object):
                 self.persistables[var.name] = var
         persistables = []
         for var in self.persistables:
-            if 'reader' not in var and 'double_buffer' not in var:
+            if 'reader' not in var and 'double_buffer' not in var and var not in self.teacher_persistables:
                 persistables.append(self.persistables[var])
 
         io.save_vars(exe.exe, path, vars=persistables)

python/paddle/fluid/contrib/slim/tests/test_graph_wrapper.py

@@ -143,6 +143,11 @@ class TestGraphWrapper(unittest.TestCase):
         self.build_program()
         self.assertEquals(self.train_graph.flops(), 354624)
 
+    def test_merge(self):
+        self.build_program()
+        self.train_graph.merge(self.eval_graph)
+        self.assertEquals(len(self.train_graph.ops()), 72)
+
 
 if __name__ == '__main__':
     unittest.main()