Created by: wangkuiyi

In class framework::OperatorBase, there are some constants:

class OperatorBase {
 public:
  /// If a variable is a empty variable, that name will be used.
  static std::string EMPTY_VAR_NAME() { return "@EMPTY@"; }

  /// If a variable is a temporary variable, that name will be set in Python,
  /// but it will be convert to a unique name in scope after OpCreator.
  static std::string TMP_VAR_NAME() { return "@TEMP@"; }

  /// If a variable's name has a certain suffix, it means that the
  /// variable is the gradient of another varibale.
  /// e.g. Variable "x@GRAD" is the gradient of varibale "x".
  static std::string GRAD_VAR_SUFFIX() { return "@GRAD"; }

  static std::string GRAD_VAR_NAME(const std::string& name) {
    return name + GRAD_VAR_SUFFIX();
  }

which are not logically in the scope of OperatorBase, but in order to put them into class OperatorBase, we intentionally defined these constants as static functions. This is unnecessary overkilling.