Handled special sum_grad_op code gen in Eager Dygraph (#38573)

* Handled special sum_grad_op code gen in Eager Dygraph

* Fixed merge issues

python/paddle/fluid/tests/unittests/test_imperative_basic.py

@@ -321,7 +321,7 @@ class TestImperative(unittest.TestCase):
                 with paddle.set_grad_enabled(True):
                     self.assertTrue(paddle.is_grad_enabled())
 
-    def test_sum_op(self):
+    def func_sum_op(self):
         x = np.ones([2, 2], np.float32)
         with fluid.dygraph.guard():
             inputs = []
@@ -338,7 +338,7 @@ class TestImperative(unittest.TestCase):
                 tmp = paddle.to_tensor(x)
                 tmp.stop_gradient = False
                 inputs2.append(tmp)
-            ret2 = fluid.layers.sums(inputs2)
+            ret2 = paddle.add_n(inputs2)
             loss2 = fluid.layers.reduce_sum(ret2)
             fluid.set_flags({'FLAGS_sort_sum_gradient': True})
             loss2.backward()
@@ -349,6 +349,11 @@ class TestImperative(unittest.TestCase):
             a = inputs2[0].gradient()
             self.assertTrue(np.allclose(inputs2[0].gradient(), x))
 
+    def test_sum_op(self):
+        with _test_eager_guard():
+            self.func_sum_op()
+        self.func_sum_op()
+
     def func_empty_var(self):
         with fluid.dygraph.guard():
             cur_program = fluid.Program()