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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def get_model_url_base():
    '''
    Returns a root folder for models.
    '''
    url_base = get_model_url_base_from_env()
    if url_base is not None:
        logger.info('NNBLA_MODELS_URL_BASE is set as {}.'.format(url_base))
    else:
        url_base = 'https://nnabla.org/pretrained-models/nnp_models/'
    return url_base