fix bug: consume in always

Former-commit-id: aa24910e

oneflow/core/graph/backward_compute_task_node.cpp

@@ -32,7 +32,7 @@ void BackwardCompTaskNode::ConsumeAllRegsts() {
     }
   }
 
-  if (GetProducedRegst("in_diff")) { ConsumeRegst("in", GetRelatedInRegst()); }
+  ConsumeRegst("in", GetRelatedInRegst());
 }
 
 void BackwardCompTaskNode::BuildExecGphAndRegst() {
@@ -88,7 +88,6 @@ void BackwardCompTaskNode::BuildActivationDiffRegst() {
 
 void BackwardCompTaskNode::BuildInDiffRegst() {
   std::shared_ptr<RegstDesc> in_diff_regst = GetProducedRegst("in_diff");
-  if (!in_diff_regst) { return; }
   std::shared_ptr<RegstDesc> in_regst = GetConsumedRegst("in");
   mut_exec_gph().ForEachNode([&](ExecNode* cur_node) {
     HashSet<std::string> found_lbns;
@@ -98,8 +97,10 @@ void BackwardCompTaskNode::BuildInDiffRegst() {
     for (const std::string& idbn : cur_node->op()->input_diff_bns()) {
       const std::string& lbn = cur_node->op()->Lbn4BnInOp(idbn);
       if (found_lbns.find(lbn) != found_lbns.end()) { continue; }
-      in_diff_regst->AddLbn(lbn);
-      cur_node->BindBnInOpAndRegst(idbn, in_diff_regst);
+      if (in_diff_regst) {
+        in_diff_regst->AddLbn(lbn);
+        cur_node->BindBnInOpAndRegst(idbn, in_diff_regst);
+      }
       cur_node->BindBnInOpAndRegst(GenUnDiffBn(idbn), in_regst);
     }
   });