.implode() returns list(list()) instead of list()

[pastastical]

Checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of Polars.

Reproducible example

data = {"a":[1,1,1], "b":[2,3,4] }
pl.DataFrame(data).groupby("a").agg(
    pl.col("b").implode()
)

shape: (1, 2)

a	b
i64	list[list[i64]]
1	[[2, 3, 4]]

Issue description

When aggregating a group column using .implode(), the result is a column of type list(list()) instead of list().

Expected behavior

data = {"a":[1,1,1], "b":[2,3,4] }
pl.DataFrame(data).groupby("a").agg(
    pl.col("b").implode()
)

shape: (1, 2)

a	b
i64	list[i64]
1	[2, 3, 4]

Installed versions

--------Version info---------
Polars:              0.18.8
Index type:          UInt32
Platform:            Windows-10-10.0.22621-SP0
Python:              3.10.4 (tags/v3.10.4:9d38120, Mar 23 2022, 23:13:41) [MSC v.1929 64 bit (AMD64)]

----Optional dependencies----
adbc_driver_sqlite:  <not installed>
cloudpickle:         <not installed>
connectorx:          <not installed>
deltalake:           0.9.0
fsspec:              <not installed>
matplotlib:          3.7.1
numpy:               1.24.3
pandas:              2.0.1
pyarrow:             12.0.0
pydantic:            <not installed>
sqlalchemy:          <not installed>
xlsx2csv:            <not installed>
xlsxwriter:          <not installed>```

</details>

[cmdlineluser]

You can just remove the .implode() - #6487

[ritchie46]

This is expected as @cmdlineluser refers to in #6487