Merge pull request #4703 from Xreki/core_optimize_backward

Simplify backward when inserting a sum operator to accumulate all duplicated variables

Author: luotao1

paddle/framework/backward.cc

@@ -172,30 +172,14 @@ static std::unique_ptr<OperatorBase> BackwardRecursive(
                               std::to_string(i));
         net->ops_[op_offset]->Rename(name, dup_outputs.back());
       }
-      // collect all the offset to append `add` op for each alias
-      //
-      // one variable is shared between multiple operators.
-      // insert add operator one by one, then add it to output
-      for (size_t output_idx = 0; output_idx < dup_outputs.size() - 1;
-           ++output_idx) {
-        auto insert_add_x = dup_outputs[output_idx];
-        auto insert_add_y = dup_outputs[output_idx + 1];
-        auto insert_add_out = name + "@SHARED@" + std::to_string(output_idx);
-        // first add op inserted
-        if (output_idx == dup_outputs.size() - 2) {
-          insert_add_out = name;
-        }
-        if (output_idx != 0) {
-          insert_add_y = name + "@SHARED@" + std::to_string(output_idx - 1);
-        }
-        insert_position.push_back(
-            {dup_op.back(),
-             OpRegistry::CreateOp("sum", {{"X", {insert_add_x, insert_add_y}}},
-                                  {{"Out", {insert_add_out}}}, {})});
-      }
+      // collect all the offset for each alias,
+      // insert a sum operator to add all aliases to output
+      insert_position.push_back(
+          {dup_op.back(), OpRegistry::CreateOp("sum", {{"X", dup_outputs}},
+                                               {{"Out", {name}}}, {})});
     }
 
-    // make sure the inserted `add` ops follow the BFS order.
+    // make sure the inserted `sum` ops follow the BFS order.
     insert_position.sort(
         [](const Pos& l, const Pos& r) { return l.first > r.first; });