MB-66396: Add IndexIVF APIs to fetch list of centroids & cardinality

[abhinavdangeti]

No description provided.

[deepkaran]

Btw, if you don't want to change faiss, you can do the reconstruct in cgo as well

// Get quantizer, nlist, d
q := C.faiss_IndexIVF_quantizer(ivf)
if q == nil {
	return nil, 0, fmt.Errorf("IVF quantizer is nil")
}
nlist := int(C.faiss_IndexIVF_nlist(ivf))
d = int(C.faiss_Index_d(q))

if nlist == 0 {
	return [][]float32{}, d, nil
}

// Reconstruct all centroids [0..nlist-1] into a flat slice
buf := make([]float32, nlist*d)
rc := C.faiss_Index_reconstruct_n(
	q,
	0,                         // i0
	C.idx_t(nlist),            // ni
	(*C.float)(unsafe.Pointer(&buf[0])),
)
if rc != 0 {
	return nil, 0, fmt.Errorf("reconstruct_n failed (rc=%d)", int(rc))
}