Implement GPU IVF add_preassigned via add_core (implement #3908)

[weiwch]

Summary

This implements GPU IVF support for ivf_tools.add_preassigned, addressing #3908.

GPU IVF already supports search_preassigned, but ivf_tools.add_preassigned previously only worked for CPU IVF indexes because GPU IVF did not expose the corresponding add_core path. This implements the follow-up I mentioned in #3908 by refactoring my earlier faiss_preassigned work into the mainline-compatible add_core path. This PR adds GpuIndexIVF::add_core(...), wires precomputed IVF list ids through the existing GPU add paging/copy path, and makes the existing ivf_tools.add_preassigned helper work for GPU IVF indexes without any change of ivf_tools.py.

The implementation follows the CPU IVF design:

• add GpuIndexIVF::add_core(...) with the same naming and argument order as IndexIVF::add_core(...)
• pass precomputed_idx through the existing GPU add paging/copy path
• add IVFBase::addVectorsPreassigned(...) for appending vectors to precomputed IVF lists

contrib.ivf_tools.add_preassigned can now call index_ivf.add_core(...) for both CPU and GPU IVF indexes. Compared with my earlier fork implementation, this does not introduce separate GPU-specific add_preassigned* APIs; the naming and entry point are aligned with the existing CPU IVF API.

Implementation note

GpuIndex::addPaged_ and addPaged_ex_ were moved from private to protected so GpuIndexIVF::add_core(...) can reuse the existing GPU add paging path.

Tests

Added a GPU IVFFlat add-preassigned test that:

• builds CPU and GPU IVF indexes
• assigns vectors with an external quantizer
• adds vectors through ivf_tools.add_preassigned
• compares CPU/GPU results using search_preassigned

[meta-codesync]

@mnorris11 has imported this pull request. If you are a Meta employee, you can view this in D111188416.

[mnorris11]

Thanks for this change!
Would you be able to add 1 small test that exercises the GpuIndexIVFPQ flow? Reason: it has by_residual = true by default.
Then it looks good to me to merge.

[weiwch]

I hadn’t used preassigned GpuIndexIVFPQ in my own project before, so I hadn’t tested it or added a test covering that flow. I’ll add a small test soon.