bugfix for sparse tensor reduce. (#52785)

paddle/fluid/distributed/collective/reducer.cc

@@ -821,9 +821,9 @@ void EagerReducer::MarkVarReady(const size_t var_index,
 
   auto &group = groups_[group_index];
   auto &group_tensor = group.dense_tensors_[inside_group_index];
-  const auto length = group.length_[inside_group_index];
 
   if (!group.is_sparse_) {
+    const auto length = group.length_[inside_group_index];
     if (is_used_var) {
       auto *autograd_meta = tensors_[var_index].get_autograd_meta();
       auto &grad_tensor =