Created by: JiayiFeng

In backward building process, @Empty@ is used to rename variables to indicate that they are not necessary for backward. If we try accessing a @Empty@, we shall get a nullptr. This is required by several places in our code, for example:

class WhileGradOpVarTypeInference : public framework::VarTypeInference {
 public:
  void operator()(const framework::OpDescBind &op_desc,
                  framework::BlockDescBind *block) const override {
    auto p_names = op_desc.Input(kParameters);
    auto pg_names = op_desc.Output(framework::GradVarName(kParameters));

    for (size_t i = 0; i < p_names.size(); ++i) {
      auto &p_var = detail::Ref(block->FindVarRecursive(p_names[i]));
      auto *g_var = block->FindVarRecursive(pg_names[i]);
      if (g_var != nullptr) {  // Gradient could be @EMPTY@
        VLOG(5) << "Setting " << pg_names[i] << " following " << p_names[i]
                << " type: " << p_var.GetType();
        g_var->SetType(p_var.GetType());
        g_var->SetDataType(p_var.GetDataType());
      }
    }
  }
};

if g_var is not nullptr while pg_names[i] is @Empty@, we wil go into the if block and then invoke g_var->SetDataType(p_var.GetDataType());. However, as a variable which has no gradient value, p_varmaight be some special type which is not supported by GetDataType() (e.g. lod_rank_table).

To aviod it, we shall make sure that Executor does not create variables whose name are @Empty@