fix fuse optimizer ops (#17102)

test=develop

paddle/fluid/framework/details/fuse_optimizer_op_pass.cc

@@ -147,6 +147,18 @@ void FuseOptimizerOpPass::InitFusedGradsAndAllocSpaceForGrads(
       vars.emplace(node->Var()->Name(), node);
     }
   }
+
+  // Set Gradients as Persistable to prevent this var becoming reusable.
+  for (auto &grad_var_name : grads) {
+    auto iter = vars.find(grad_var_name);
+    PADDLE_ENFORCE(iter != vars.end());
+    PADDLE_ENFORCE_NOT_NULL(iter->second->Var());
+    PADDLE_ENFORCE(iter->second->Var()->GetType() == proto::VarType::LOD_TENSOR,
+                   "Currently the gradient type only should be LoDTensor when "
+                   "fusing optimizer ops.");
+    iter->second->Var()->SetPersistable(true);
+  }
+
   // Init Grads
   for (auto it = local_scopes.rbegin(); it != local_scopes.rend(); ++it) {
     auto &scope = *it;
@@ -154,13 +166,10 @@ void FuseOptimizerOpPass::InitFusedGradsAndAllocSpaceForGrads(
     PADDLE_ENFORCE(scope->FindVar(fused_grad_name) == nullptr,
                    "%s has existed in scope.", fused_grad_name);
     scope->Var(fused_grad_name)->GetMutable<LoDTensor>();
-
     for (auto &grad_var_name : grads) {
       auto iter = vars.find(grad_var_name);
       PADDLE_ENFORCE(iter != vars.end());
       PADDLE_ENFORCE_NOT_NULL(iter->second->Var());
-      PADDLE_ENFORCE_EQ(iter->second->Var()->GetType(),
-                        proto::VarType::LOD_TENSOR);
       scope->Var(grad_var_name)->GetMutable<LoDTensor>();
     }
   }