support backward return None, when corresponding input tensor without gradient (#32494)

* support backward return None.

* edit unittest.

* edit code according to CI

* Improve error information

paddle/fluid/operators/py_layer_op.cc

@@ -60,33 +60,51 @@ void RunPyObject(py::object *py_object,
           outs->size(), result_tuple.size()));
     }
     for (size_t i = 0; i < result_tuple.size(); i++) {
+      if ((*outs)[i] != nullptr) {
         if (Py_None != result_tuple[i].ptr()) {
           try {
             auto result_var =
                 result_tuple[i].cast<std::shared_ptr<imperative::VarBase>>();
             *(*outs)[i] = result_var->Var();
           } catch (py::cast_error &) {
-          PADDLE_THROW(platform::errors::Unimplemented(
+            PADDLE_THROW(platform::errors::InvalidArgument(
                 "The output of `PyLayer.backward` should be `Tensor`."));
           }
         } else {
-        PADDLE_THROW(platform::errors::Unimplemented(
-            "The output of `PyLayer.backward` can not be `None`."));
+          PADDLE_THROW(platform::errors::InvalidArgument(
+              "The %dth input tensor of forward needs gradient and the "
+              "corresponding gradient cannot be None.",
+              i));
+        }
+      } else {
+        if (Py_None != result_tuple[i].ptr()) {
+          PADDLE_THROW(platform::errors::InvalidArgument(
+              "The %dth input tensor of forward do not need gradient and the "
+              "corresponding gradient should be `None`.",
+              i));
+        }
       }
     }
   } else {
+    if ((*outs)[0] != nullptr) {
       if (Py_None != py_result.ptr()) {
         try {
           auto result_var =
               py_result.cast<std::shared_ptr<imperative::VarBase>>();
           *((*outs)[0]) = result_var->Var();
         } catch (py::cast_error &) {
-        PADDLE_THROW(platform::errors::Unimplemented(
+          PADDLE_THROW(platform::errors::InvalidArgument(
               "The output of `PyLayer.backward` should be `Tensor`."));
         }
       } else {
-      PADDLE_THROW(platform::errors::Unimplemented(
-          "The output of `PyLayer.backward` can not be `None`."));
+        PADDLE_THROW(platform::errors::InvalidArgument(
+            "The input tensor of forward needs gradient, so the output of "
+            "`PyLayer.backward` can not be `None`."));
+      }
+    } else {
+      PADDLE_THROW(platform::errors::InvalidArgument(
+          "The input tensor of forward do not need gradient, so the output of "
+          "`PyLayer.backward` should be `None`."));
     }
   }
 }

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

@@ -52,6 +52,40 @@ class TestPyLayer(unittest.TestCase):
 
         self.assertTrue(np.max(np.abs((input1.grad - input2.grad))) < 1e-10)
 
+    def test_simple_pylayer_return_none_with_no_grad(self):
+        class tanh(PyLayer):
+            @staticmethod
+            def forward(ctx, x1, x2, func1, func2=paddle.square):
+                ctx.func = func2
+                y1 = func1(x1)
+                y2 = func1(x2)
+                ctx.save_for_backward(y1, y2)
+                return y1, y2
+
+            @staticmethod
+            def backward(ctx, dy1, dy2):
+                y1, y2 = ctx.saved_tensor()
+                re1 = dy1 * (1 - ctx.func(y1))
+                re2 = dy2 * (1 - paddle.square(y2))
+                return re1, None
+
+        input1 = paddle.randn([2, 3]).astype("float64")
+        input2 = input1.detach().clone()
+        input3 = input1.detach().clone()
+        input4 = input1.detach().clone()
+        input1.stop_gradient = False
+        input2.stop_gradient = False
+        input3.stop_gradient = True
+        input4.stop_gradient = True
+        z = tanh.apply(input1, input3, paddle.tanh, paddle.square)
+        z = z[0] + z[1]
+        z.mean().backward()
+
+        z2 = paddle.tanh(input2) + paddle.tanh(input4)
+        z2.mean().backward()
+
+        self.assertTrue(np.max(np.abs((input1.grad - input2.grad))) < 1e-10)
+
     def test_simple_pylayer_single_output(self):
         class tanh(PyLayer):
             @staticmethod
@@ -196,7 +230,7 @@ class TestPyLayer(unittest.TestCase):
         input2.stop_gradient = False
         z = Layer_bk_none1.apply(input2)
 
-        with self.assertRaises(NotImplementedError):
+        with self.assertRaises(ValueError):
             with paddle.fluid.dygraph.guard():
                 z.sum().backward()
 
@@ -212,7 +246,7 @@ class TestPyLayer(unittest.TestCase):
         input1 = paddle.randn([2, 3]).astype("float64")
         input1.stop_gradient = False
         z = Layer_bk_none2.apply(input1, input1)
-        with self.assertRaises(NotImplementedError):
+        with self.assertRaises(ValueError):
             with paddle.fluid.dygraph.guard():
                 z.mean().backward()
 
@@ -228,14 +262,14 @@ class TestPyLayer(unittest.TestCase):
         input1 = paddle.randn([2, 3]).astype("float64")
         input1.stop_gradient = False
         z = Layer_bk_one1.apply(input1)
-        with self.assertRaises(NotImplementedError):
+        with self.assertRaises(ValueError):
             with paddle.fluid.dygraph.guard():
                 z.mean().backward()
 
         class Layer_bk_one2(PyLayer):
             @staticmethod
-            def forward(ctx, x):
-                return x * 2, x * 5
+            def forward(ctx, x1, x2):
+                return x1 * 2, x2 * 5
 
             @staticmethod
             def backward(ctx, *args):
@@ -243,8 +277,9 @@ class TestPyLayer(unittest.TestCase):
 
         input1 = paddle.randn([2, 3]).astype("float64")
         input1.stop_gradient = False
-        z = Layer_bk_one1.apply(input1)
-        with self.assertRaises(NotImplementedError):
+        y = Layer_bk_one2.apply(input1, input1)
+        z = y[0] + y[1]
+        with self.assertRaises(ValueError):
             with paddle.fluid.dygraph.guard():
                 z.mean().backward()
 
@@ -279,6 +314,45 @@ class TestPyLayer(unittest.TestCase):
                 z = z[0] + z[1]
                 z.mean().backward()
 
+    def test_pylayer_bk_return_none(self):
+        class Layer_bk_none1(PyLayer):
+            @staticmethod
+            def forward(ctx, x1, x2):
+                return x1 + x2
+
+            @staticmethod
+            def backward(ctx, dy):
+                return 1
+
+        input1 = paddle.randn([2, 3]).astype("float64")
+        input2 = paddle.randn([2, 3]).astype("float64")
+        input1.stop_gradient = True
+        input2.stop_gradient = False
+        z = Layer_bk_none1.apply(input1, input2)
+
+        with self.assertRaises(ValueError):
+            with paddle.fluid.dygraph.guard():
+                z.mean().backward()
+
+        class Layer_bk_none2(PyLayer):
+            @staticmethod
+            def forward(ctx, x1, x2):
+                return x1 * 2, x2 * 5
+
+            @staticmethod
+            def backward(ctx, *args):
+                return 1, 1
+
+        input1 = paddle.randn([2, 3]).astype("float64")
+        input2 = paddle.randn([2, 3]).astype("float64")
+        input1.stop_gradient = True
+        input2.stop_gradient = False
+        z = Layer_bk_none2.apply(input1, input2)
+        z = z[0] + z[1]
+        with self.assertRaises(ValueError):
+            with paddle.fluid.dygraph.guard():
+                z.mean().backward()
+
     def test_pylayer_inplace(self):
         class cus_tanh(PyLayer):
             @staticmethod