add more error info for these ops without double grad ops. (#25987)

paddle/fluid/imperative/partial_grad_engine.cc

@@ -887,7 +887,10 @@ void PartialGradTask::RunEachOp(OpBase *op) {
                                              op->Attrs(), op->place());
     PADDLE_ENFORCE_NOT_NULL(
         double_grad_node,
-        platform::errors::NotFound("The Op %s doesn't have any grad op.",
+        platform::errors::NotFound("The Op %s doesn't have any grad op. If you "
+                                   "don't intend calculating higher order "
+                                   "derivatives, please set `create_graph` to "
+                                   "False.",
                                    op->Type()));
     VLOG(10) << "Create " << double_grad_node->size()
              << " double grad op(s) for " << op->Type()

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

@@ -298,15 +298,16 @@ class TestDygraphDoubleGradSortGradient(TestDygraphDoubleGrad):
 
 class TestDygraphDoubleGradVisitedUniq(TestCase):
     def test_compare(self):
-        value = np.random.uniform(-1, 1, [10, 3, 32, 32]).astype('float32')
+        value = np.random.uniform(-0.5, 0.5, 100).reshape(10, 2,
+                                                          5).astype("float32")
 
         def model_f(input):
-            conv2d = fluid.dygraph.Conv2D(3, 2, 3)
+            linear = fluid.dygraph.Linear(5, 3, bias_attr=False)
             for i in range(10):
                 if i == 0:
-                    out = conv2d(input)
+                    out = linear(input)
                 else:
-                    out = out + conv2d(input)
+                    out = out + linear(input)
             return out
 
         backward_strategy = fluid.dygraph.BackwardStrategy()
@@ -321,8 +322,7 @@ class TestDygraphDoubleGradVisitedUniq(TestCase):
             dx = fluid.dygraph.grad(
                 outputs=[out],
                 inputs=[a],
-                create_graph=True,
-                retain_graph=True,
+                create_graph=False,
                 only_inputs=True,
                 allow_unused=False,
                 backward_strategy=backward_strategy)
@@ -339,9 +339,25 @@ class TestDygraphDoubleGradVisitedUniq(TestCase):
 
             grad_2 = a.gradient()
 
-        self.assertTrue(
-            np.allclose(
-                grad_1, grad_2, rtol=1.e-5, atol=1.e-8, equal_nan=True))
+        self.assertTrue(np.array_equal(grad_1, grad_2))
+
+
+class TestRaiseNoDoubleGradOp(TestCase):
+    def raise_no_grad_op(self):
+        with fluid.dygraph.guard():
+            x = fluid.layers.ones(shape=[2, 3, 2, 2], dtype='float32')
+            x.stop_gradient = False
+            y = paddle.fluid.layers.batch_norm(x)
+
+            dx = fluid.dygraph.grad(
+                outputs=[y], inputs=[x], create_graph=True,
+                retain_graph=True)[0]
+
+            loss = fluid.layers.reduce_mean(dx)
+            loss.backward()
+
+    def test_raise(self):
+        self.assertRaises(fluid.core.EnforceNotMet, self.raise_no_grad_op)
 
 
 if __name__ == '__main__':