[FEA] IVF-PQ Build Factories for Precomputed Centroids and Codebooks

[tarang-jain]

IVF-PQ Index Build API Enhancements and PIMPL Refactoring

Summary

This PR adds new build APIs for IVF-PQ indices using precomputed centroids and implements a complete PIMPL refactoring with owning/view semantics

Key Changes

1. New Build APIs for Precomputed Centroids
   Added cuvs::neighbors::ivf_pq::build() overloads that accept precomputed cluster centroids, PQ codebooks, and rotation matrices
   Enables building indices from pre-trained models without re-training
   Supports both device and host input data with automatic memory transfer
2. PIMPL Refactoring with Owning/View Semantics
   owning_impl: Owns centroid and codebook data (traditional behavior)
   view_impl: References external centroid data without copying
   Maintains identical search behavior with zero data copying
3. Enhanced Helper Functions
   Removed mutator functions that directly modify the state of the index. Instead we have helpers for the user to fetch and own the transformed data

View indices avoid copying large centroid arrays
Backward Compatibility: All existing APIs work unchanged

[Updates] (as of 12/23/2025):
The non-const getters that allow direct modification of the state of the index:
pq_centers(), centers(), centers_rot() and rotation_matrix() have been removed from the interface and the user-facing class.

[copy-pr-bot]

Auto-sync is disabled for draft pull requests in this repository. Workflows must be run manually.

Contributors can view more details about this message here.

[tarang-jain]

/ok to test 2aae6c1

[tarang-jain]

/ok to test 9d2dc10

[lowener]

LGTM

[tarang-jain]

/ok to test 94a88d9

[tarang-jain]

@lowener the current approach is causing test failures. By default, when doing something like index.pq_centers(), the non-const getters get called, and we throw errors there in the view_impl. Furthermore, I have had to change the noexcept status in the interface for the non-const getters. I personally don't see a problem with allowing the user to access and modify their own data. I think something like this:
std::as_const(index).pq_centers() would force the const getters to get called, but having to do this for only view type indexes would beat purpose of the polymorphism.

[lowener]

LGTM, having the non-const getter not accessible in the virtual interface should make it less prone to errors for end-users.

[tarang-jain]

/ok to test 5921585

[tarang-jain]

/ok to test e9aaca3

[tarang-jain]

/ok to test fc3e4eb

[tarang-jain]

/ok to test b95c5ff

[tarang-jain]

/merge