Set the attribute of the Booster.

        Parameters
        ----------
        **kwargs
            The attributes to set. Setting a value to None deletes an attribute.

def set_attr(self, **kwargs):
        """Set the attribute of the Booster.

        Parameters
        ----------
        **kwargs
            The attributes to set. Setting a value to None deletes an attribute.
        """
        for key, value in kwargs.items():
            if value is not None:
                if not isinstance(value, STRING_TYPES):
                    raise ValueError("Set Attr only accepts string values")
                value = c_str(str(value))
            _check_call(_LIB.XGBoosterSetAttr(
                self.handle, c_str(key), value))