Fix clone function of Program to avoid memory leak. (#10358)

* Fix clone function of Program to avoid memory leak.

* Fix inference_optimize function of framework.py.

* Reuse inference_optimize in framework.py.

* Add comments.

Author: wanghaoshuang

python/paddle/fluid/framework.py

@@ -1042,13 +1042,14 @@ def clone(self, for_test=False):
         Returns(Program):
             The cloned Program object.
         """
-        p = Program()
         if for_test:
-            p.desc = core.inference_optimize(self.desc)
+            p = self.inference_optimize()
         else:
+            p = Program()
             p.desc = core.ProgramDesc(self.desc)
-        p.blocks = [Block(p, i) for i in xrange(self.desc.num_blocks())]
-        p.sync_with_cpp()
+            p.blocks = [Block(p, i) for i in xrange(self.desc.num_blocks())]
+            p.sync_with_cpp()
+
         p.copy_param_info_from(self)
         return p
 
@@ -1061,7 +1062,7 @@ def prune(self, targets):
                 if isinstance(t, Variable):
                     # After transpiler processing, the op that output this
                     # variable maybe has been changed, so t.op is not reliable
-                    # and we need to find the current op that generate this 
+                    # and we need to find the current op that generate this
                     # variable here.
                     t.op = None
                     global_block = self.global_block()
@@ -1087,8 +1088,16 @@ def prune(self, targets):
         return res
 
     def inference_optimize(self):
+        # this is an alternative implement before
+        # core.inference_optimize being fixed.
         res = Program()
-        res.desc = core.inference_optimize(self.desc)
+        res.desc = core.ProgramDesc(self.desc)
+        for i in xrange(res.desc.num_blocks()):
+            block = res.desc.block(i)
+            for j in xrange(block.op_size()):
+                op = block.op(j)
+                if op.has_attr('is_test'):
+                    op.set_attr('is_test', True)
         res.blocks = [Block(res, i) for i in xrange(res.desc.num_blocks())]
         res.sync_with_cpp()
         return res