fix mul double grad (#20040)

paddle/fluid/operators/mul_op.cc

@@ -244,7 +244,8 @@ class MulDoubleGradOp : public framework::OperatorWithKernel {
     PADDLE_ENFORCE(ctx->HasInput("Y"), "Input(Y) should not be null");
     PADDLE_ENFORCE(ctx->HasInput("DOut"), "Input(DOut) should not be null");
 
-    if (ctx->HasOutput("DDOut") && ctx->HasInput("DDX")) {
+    if (ctx->HasOutput("DDOut") &&
+        (ctx->HasInput("DDX") || (ctx->HasInput("DDY")))) {
       ctx->ShareDim("DOut", "DDOut");
     }
     if (ctx->HasOutput("DX") && ctx->HasInput("DDY")) {
@@ -275,9 +276,9 @@ class MulDoubleGradMaker : public framework::SingleGradOpDescMaker {
     auto ddw = OutputGrad(framework::GradVarName("Y"));
     std::vector<std::string> empty_str = {};
 
-    retv->SetOutput("DDOut", (ddx.empty())
-                                 ? empty_str
-                                 : InputGrad(framework::GradVarName("Out")));
+    if (!ddx.empty() || !ddw.empty()) {
+      retv->SetOutput("DDOut", InputGrad(framework::GradVarName("Out")));
+    }
     retv->SetOutput("DX", ddw.empty() ? empty_str : InputGrad("X"));
     retv->SetOutput("DY", ddx.empty() ? empty_str : InputGrad("Y"));