Function train_test_split 

pub fn train_test_split(
    data: &Dataset,
    test_ratio: f64,
    seed: u64,
) -> (Dataset, Dataset)

Split a dataset into training and test sets.

test_ratio should be between 0.0 and 1.0 (e.g., 0.2 for 80/20 split). The seed controls the random shuffle for reproducibility.