[FEA] Support non-broadcasting (non-scalar) assignment on list columns

[wence-]

Consider

import cudf

s = cudf.Series([[1], [2], [3]])
g = cudf.Series([[4], [5]])

s.iloc[:2] = g

I would expect this to work and produce [[4], [5], [3]]. Instead, we get an error:

ValueError: Can not set <cudf.core.column.lists.ListColumn object at 0x7fea4b0f5cc0>
[
  [
    4
  ],
  [
    5
  ]
]
dtype: list into ListColumn

Indeed, the only things we can __setitem__ into list columns are cudf.NA and a singleton list (treated as a cudf.Scalar) that is broadcast to all entries.

[GregoryKimball]

Is this topic covered by another issue in #12793?

[wence-]

Maybe related to #11944 ?