Preprocessing roll fill_strategy NameError

[miroslaavi]

Describe the bug A tiny bug/typo in the preprocessing roll method for the fill strategy leading to NameError when using roll function. The reason is that the variable name used is X_X_rolling instead of X_rolling in the row 324.

To Reproduce

import polars as pl
from functime.preprocessing import roll
from pathlib import Path

file_path = Path(__file__).parent.parent / "data" / "commodities.parquet"

df = pl.read_parquet(source=file_path)

transformer = roll(
    window_sizes=[10, 30, 60],
    stats=["sum", "mean", "std"],
    freq="1mo",
    fill_strategy="backward",
)

df.pipe(transformer).collect()