Function encode_rows_with_layout 

pub fn encode_rows_with_layout(
    by: &[Column],
    layout: &RowEncodingLayout,
    propagate_nulls: bool,
) -> Result<Series, PolarsError>

Encodes by into rows of layout, which must be made for the dtypes of by. With propagate_nulls a row is null if any of its values is null.