Format feature name for hashed features.

def _format_unhashed_feature(name, hl_spaces, sep=' | '):
    # type: (List, bool, str) -> str
    """
    Format feature name for hashed features.
    """
    return sep.join(
        format_signed(n, _format_single_feature, hl_spaces=hl_spaces)
        for n in name)