GroupBy.idxmin and idxmax are very broken

[phofl]

The implementation is broken as soon as the max or min value is not in the first partition

This was broken for years, so only adding for API compatibility now but we should fix this

def test_df_groupby_idxmax():
    pdf = pd.DataFrame(
        {"idx": list(range(4)), "group": [1, 1, 2, 2, 1], "value": [10, 20, 20, 10, 40]}
    ).set_index("idx")

    ddf = dd.from_pandas(pdf, npartitions=3)

    expected = pd.DataFrame({"group": [1, 2], "value": [4, 2]}).set_index("group")

    result_pd = pdf.groupby("group").idxmax()
    result_dd = ddf.groupby("group").idxmax()

    assert_eq(result_pd, result_dd)
    assert_eq(expected, result_dd)

This is a very simple reproducer for the underlying issue