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.

python/paddle/fluid/framework.py

@@ -1042,13 +1042,14 @@ class Program(object):
         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.copy_param_info_from(self)
         return p
 
@@ -1087,8 +1088,16 @@ class Program(object):
         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