[BUG] in polars-gpu, group_by(maintain_order=True) is not ordered

[KazukiOnodera]

Steps/Code to reproduce bug

import polars as pl
import numpy as np

df = pl.DataFrame(
    {
        "random": np.random.rand(30_000),
        "groups": np.random.randint(100, size=30_000),
    }
)
df = df.lazy()
 
df.group_by("groups", maintain_order=True).agg(pl.col("random").sum()).collect(engine="gpu")

df.group_by("groups", maintain_order=True).agg(pl.col("random").sum()).collect()

Expected behavior
The result should be same as cpu.

Environment details

• cudf-cu12-24.8.3
• cudf-polars-cu12-24.8.3
• cupy-cuda12x-13.3.0
• polars-1.8.1
• rmm-cu12-24.8.2