fix untime fail for output var stop_gradient=True for fusion group (#23317)

paddle/fluid/framework/ir/fusion_group/code_generator.cc

@@ -99,6 +99,7 @@ std::vector<OperationExpression> CodeGenerator::ConvertToExpressions(
           input_ids.push_back(-1);
         }
       }
+
       // Output ids should be set in fixed order, like:
       //  - dx, dy in backward operations
       std::vector<int> output_ids;
@@ -106,10 +107,6 @@ std::vector<OperationExpression> CodeGenerator::ConvertToExpressions(
           OperationMap::Instance().Get(op->Type()).output_names;
 
       for (auto& name : output_names) {
-        PADDLE_ENFORCE_EQ(
-            op->Output(name).size(), 1U,
-            platform::errors::InvalidArgument(
-                "Output(%s) of operation %s is not set.", name, op->Type()));
         PADDLE_ENFORCE_NE(
             var_ids.find(op->Output(name)[0]), var_ids.end(),
             platform::errors::InvalidArgument(

paddle/fluid/framework/ir/fusion_group/elementwise_group_detector.cc

@@ -111,6 +111,15 @@ bool ElementwiseGroupDetector::IsElementwiseOp(const Node* n) {
         }
       }
     }
+
+    auto op = n->Op();
+    std::vector<std::string> output_names =
+        OperationMap::Instance().Get(op->Type()).output_names;
+
+    for (auto& name : output_names) {
+      if (op->Output(name).size() != 1) return false;
+    }
+
     return true;
   }
   return false;

python/paddle/fluid/tests/unittests/ir/test_ir_fusion_group_pass.py

@@ -109,11 +109,9 @@ class FusionGroupPassTest2(FusionGroupPassTest):
             tmp_2 = layers.relu(layers.sigmoid(self.feed_vars[3]))
             tmp_3 = layers.mul(tmp_1, tmp_2)
 
-        # TODO(wangchaochaohu): support the case when some vars are set
-        #  stop_gradient = True.
-
+        self.append_gradients(tmp_3)
         self.num_fused_ops = 2
-        self.fetch_list = [tmp_3]
+        self.fetch_list = [tmp_3, self.grad(tmp_1)]
 
 
 class FusionGroupPassTestFP64(FusionGroupPassTest):