`create_hf_dataset` optional argument `model_n_ctx` is not optional

[stefan-apollo]

def create_hf_dataset(
    dataset_config: HFDatasetConfig, model_n_ctx: Optional[int] = None
) -> Dataset:
    """[...]
        model_n_ctx (int): The max context length of the model. Used for HFDatasetConfigs. Data
            sequences are packed to dataset_config.n_ctx if it is not None and is <= model_n_ctx,
            otherwise to model_n_ctx.[...]
    """
    assert model_n_ctx is not None

assert model_n_ctx is not None fails when model_n_ctx is None, and it's at the first line of the function.