Create a bundle of selected `models`.

def bundle_models(models):
    """Create a bundle of selected `models`. """
    custom_models = _get_custom_models(models)
    if custom_models is None:
        return None

    key = calc_cache_key(custom_models)
    bundle = _bundle_cache.get(key, None)
    if bundle is None:
        try:
            _bundle_cache[key] = bundle = _bundle_models(custom_models)
        except CompilationError as error:
            print("Compilation failed:", file=sys.stderr)
            print(str(error), file=sys.stderr)
            sys.exit(1)
    return bundle