Return a incremental temporary file name. The file is not created.

        Args:
            suffix (str): The suffix of the temp file.
            prefix (str): The prefix of the temp file.
            directory_name (str) : The base directory of the temp file.

        Returns:
            A string of file name. If there existing a file having
                the same name, the returned name will look like
                "{directory_name}/{prefix}.{unique_index}{suffix}"

def _make_inc_temp(self, suffix="", prefix="", directory_name="/tmp/ray"):
        """Return a incremental temporary file name. The file is not created.

        Args:
            suffix (str): The suffix of the temp file.
            prefix (str): The prefix of the temp file.
            directory_name (str) : The base directory of the temp file.

        Returns:
            A string of file name. If there existing a file having
                the same name, the returned name will look like
                "{directory_name}/{prefix}.{unique_index}{suffix}"
        """
        directory_name = os.path.expanduser(directory_name)
        index = self._incremental_dict[suffix, prefix, directory_name]
        # `tempfile.TMP_MAX` could be extremely large,
        # so using `range` in Python2.x should be avoided.
        while index < tempfile.TMP_MAX:
            if index == 0:
                filename = os.path.join(directory_name, prefix + suffix)
            else:
                filename = os.path.join(directory_name,
                                        prefix + "." + str(index) + suffix)
            index += 1
            if not os.path.exists(filename):
                # Save the index.
                self._incremental_dict[suffix, prefix, directory_name] = index
                return filename

        raise FileExistsError(errno.EEXIST,
                              "No usable temporary filename found")