[BUG] Method TSDataset.tsdataset_idx_slice loses hierarchical structure

[d-a-bunin]

🐛 Bug Report

Method TSDataset.tsdataset_idx_slice loses hierarchical structure.

Expected behavior

It shouldn't

How To Reproduce

from etna.datasets import TSDataset
from etna.datasets import generate_hierarchical_df


def main():
    df = generate_hierarchical_df(periods=100, n_segments=[2, 4], start_time="2021-01-01",)
    df, hierarchical_structure = TSDataset.to_hierarchical_dataset(df=df, level_columns=["level_0", "level_1"])
    ts = TSDataset(df=df, freq="D", hierarchical_structure=hierarchical_structure)

    new_ts = ts.tsdataset_idx_slice(start_idx=10, end_idx=50)
    assert new_ts.hierarchical_structure is not None


if __name__ == "__main__":
    main()

Environment

No response

Additional context

No response

Checklist

• Bug appears at the latest library version