Implement ST_norm_from_LUT for AdditiveQuantizer search

[mdouze]

Summary

The additive quantizers need to carry around the norms of the vectors to compute L2 distances, or they can be computed.
The storage options work but the ST_norm_from_LUT is not implemented (it is described in eq (16) of the Faiss paper).
the symptom is a crash because the interpreter does not propagate the exception correctly.

import faiss
from faiss.contrib.datasets import SyntheticDataset

ds = SyntheticDataset(20, 1000, 100, 100)
index = faiss.index_factory(ds.d, "RQ2x4")
index.rq.search_type = faiss.AdditiveQuantizer.ST_norm_from_LUT
index.train(ds.get_train())
index.add(ds.get_database())
faiss.omp_set_num_threads(1) # otherwise crashes the interpreter
index.search(ds.get_queries(), 10)

result:

RuntimeError: Error in virtual void faiss::IndexAdditiveQuantizer::search(faiss::idx_t, const float*, faiss::idx_t, float*, faiss::idx_t*, const faiss::SearchParameters*) const at [/home/circleci/miniconda/conda-bld/faiss-pkg_1709244517602/work/faiss/IndexAdditiveQuantizer.cpp:284](http://localhost:8921/home/circleci/miniconda/conda-bld/faiss-pkg_1709244517602/work/faiss/IndexAdditiveQuantizer.cpp#line=283): search type 2 not supported

[mdouze]

This is more a note for myself to implement it.