add details. test=develop

paddle/fluid/operators/elementwise/elementwise_op.h

@@ -264,6 +264,20 @@ class ElementwiseOpInplace : public framework::InplaceInToOut {
   }
 };
 
+class ElementwiseGradOpInplace : public framework::InplaceInToOut {
+ public:
+  using framework::InplaceInToOut::InplaceInToOut;
+
+ protected:
+  std::unordered_map<std::string, std::string> Apply(
+      const framework::OpDesc &op_desc,
+      framework::BlockDesc *block) const override {
+    return std::unordered_map<std::string, std::string>{
+        {framework::GradVarName("Out"), framework::GradVarName("X")},
+    };
+  }
+};
+
 }  // namespace operators
 }  // namespace paddle
 
@@ -316,4 +330,5 @@ class ElementwiseOpInplace : public framework::InplaceInToOut {
                     op_type##GradMaker,                                \
                     ::paddle::operators::ElementwiseOpInplace);        \
   REGISTER_OPERATOR(op_type##_grad,                                    \
-                    ::paddle::operators::ElementwiseOpExplicitGrad)
+                    ::paddle::operators::ElementwiseOpExplicitGrad,    \
+                    ::paddle::operators::ElementwiseGradOpInplace)