code: def set_attr(self, **kwargs):
          
          for key, value in kwargs.items():
              if value is not None:
                  if not isinstance(value, string_type):
                      raise ValueError("Only string values are accepted")
                  self.__attr[key] = value
              else:
                  self.__attr.pop(key, None)
          return self
docs: Set attributes to the Booster.
  
          Parameters
          ----------
          **kwargs
              The attributes to set.
              Setting a value to None deletes an attribute.
  
          Returns
          -------
          self : Booster
              Booster with set attributes.