def normalize_start(start, dim_size):
    """
    Normalize `start` according to the number of dimensions (`dim_size`).
    If the number of dimensions is not given, return the original input directly.
    """
    if start is None:
        return 0
    if start < 0:
        return 0 if start < -dim_size else start % dim_size
    return start if start < dim_size else dim_size