[Dy2St]Fix cond_block_grad error when handle no need grad vras (#43034)

* Fix cond_block_grad error when handle no need grad vras

* Add comment and UT

paddle/fluid/operators/controlflow/conditional_block_op.cc

@@ -153,7 +153,7 @@ class ConditionalBlockGradOp : public ConditionalOp {
                /* keep_kid_scopes */ false);
 
       AssignLocalGradientToParentScope(dev_place, cur_scope, scope,
-                                       inside_grads, outside_grads);
+                                       inside_grads, outside_grads, inputs);
       return;
     }
 
@@ -165,27 +165,36 @@ class ConditionalBlockGradOp : public ConditionalOp {
       const platform::Place &place, const framework::Scope &cur_scope,
       const framework::Scope &parent_scope,
       const std::vector<std::string> &inside_grads,
-      const std::vector<std::string> &outside_grads) const {
+      const std::vector<std::string> &outside_grads,
+      const std::vector<std::string> &inputs) const {
+    std::vector<std::string> assign_zero_outside_grads;
+    std::vector<std::string> assign_zero_inputs;
     for (size_t i = 0; i < outside_grads.size(); ++i) {
       const std::string &outside_grad_name = outside_grads[i];
       const std::string &inside_grad_name = inside_grads[i];
       VLOG(4) << "inside_grad_name = " << inside_grad_name
               << ", outside_grad_name = " << outside_grad_name;
-      framework::Variable *inside_var =
-          cur_scope.FindLocalVar(inside_grad_name);
-      if (inside_var == nullptr) {
-        continue;
-      }
       framework::Variable *outside_var =
           parent_scope.FindVar(outside_grad_name);
       if (outside_var == nullptr) {
         continue;
       }
+      framework::Variable *inside_var =
+          cur_scope.FindLocalVar(inside_grad_name);
+      if (inside_var == nullptr) {
+        assign_zero_outside_grads.emplace_back(outside_grad_name);
+        assign_zero_inputs.emplace_back(inputs[i]);
+        continue;
+      }
       platform::DeviceContext *dev_ctx =
           platform::DeviceContextPool::Instance().Get(place);
       framework::VisitVarType(*inside_var,
                               AssignFunctor(outside_var, *dev_ctx));
     }
+    // Assign zero to the grad_vars that are in outside_grads but not in
+    // inside_grads
+    AssignZeroToParentScope(place, parent_scope, assign_zero_inputs,
+                            assign_zero_outside_grads);
   }
 
   void AssignZeroToParentScope(

python/paddle/fluid/tests/unittests/dygraph_to_static/test_ifelse.py

@@ -424,6 +424,41 @@ class TestDy2StIfElseRetInt4(TestDy2StIfElseRetInt1):
         ProgramTranslator().enable(False)
 
 
+class IfElseNet(paddle.nn.Layer):
+    def __init__(self):
+        super(IfElseNet, self).__init__()
+        self.param = self.create_parameter(
+            shape=[3, 2], dtype='float32', is_bias=False)
+
+    @paddle.jit.to_static
+    def forward(self, a, b, c):
+        a = paddle.matmul(a, self.param)
+        a = paddle.reshape(a, (2, 4))
+        cond = paddle.to_tensor([10])
+        if cond == 10:
+            a_argmax = a.argmax(axis=-1)
+            b = b + self.param
+        else:
+            print(c)
+        return b
+
+
+class TestDy2StIfElseBackward(unittest.TestCase):
+    def test_run_backward(self):
+        a = paddle.randn((4, 3), dtype='float32')
+        a.stop_gradient = False
+        b = paddle.to_tensor([10]).astype('float32')
+        b.stop_gradient = False
+        c = paddle.to_tensor([2])
+        c.stop_gradient = False
+
+        net = IfElseNet()
+        net.train()
+        out = net(a, b, c)
+        out.backward()
+        self.assertTrue(np.allclose((b + net.param).numpy(), out.numpy()))
+
+
 if __name__ == '__main__':
     with paddle.fluid.framework._test_eager_guard():
         unittest.main()