Add pylibcudf.gpumemoryview support for len()/nbytes

[pentschev]

Description

Add support for len() and nbytes in pylibcudf.gpumemoryview. Having those methods is helpful to ensure proper serialization in Dask/Distributed, as utility methods that serialize objects, in this case used by cudf-polars, may use the appropriate method or property to determine the size of the object being transferred.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[Matt711]

Thanks @pentschev, can you update the type stub? https://github.com/rapidsai/cudf/blob/branch-25.04/python/pylibcudf/pylibcudf/gpumemoryview.pyi

[rjzamora]

I definitely prefer this over rapidsai/rapids-dask-dependency#89

[pentschev]

Thanks @pentschev, can you update the type stub? https://github.com/rapidsai/cudf/blob/branch-25.04/python/pylibcudf/pylibcudf/gpumemoryview.pyi

Sure, thanks for the reminder @Matt711 ! Done in 41844b7 .

[pentschev]

I definitely prefer this over rapidsai/rapids-dask-dependency#89

Agreed, closed that in favor of the change in this PR.