Applies a flatmap operator to the stream.

        Attributes:
             flatmap_fn (function): The user-defined logic of the flatmap
             (e.g. split()).

def flat_map(self, flatmap_fn):
        """Applies a flatmap operator to the stream.

        Attributes:
             flatmap_fn (function): The user-defined logic of the flatmap
             (e.g. split()).
        """
        op = Operator(
            _generate_uuid(),
            OpType.FlatMap,
            "FlatMap",
            flatmap_fn,
            num_instances=self.env.config.parallelism)
        return self.__register(op)