Fix IVF factory leak

[jbajic]

The alubsan leak happens when we have constructed the IVF,PQ3 with the broken quantization and the quantizer get released, and then the IVF construction throws and does not clean up the quantizer. The leak looks like this:

==88601==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 128 byte(s) in 1 object(s) allocated from:
    #0 0x74b254616222 in operator new(unsigned long) ../../../../src/libsanitizer/lsan/lsan_interceptors.cpp:248
    #1 0x588a0cf4c4d7 in faiss::(anonymous namespace)::parse_coarse_quantizer(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int, faiss::MetricType, std::vector<std::unique_ptr<faiss::Index, std::default_delete<faiss::Index> >, std::allocator<std::unique_ptr<faiss::Index, std::default_delete<faiss::Index> > > >&, unsigned long&, bool&) [clone .isra.0] (/home/jbajic/Projects/faiss/ivf_quantizer_leak+0xac4d7) (BuildId: 45a0934ffe20c59ae7f414fa8f8845dcec85cc8b)
    #2 0x588a0cf55524 in faiss::(anonymous namespace)::index_factory_sub(int, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, faiss::MetricType, bool) (/home/jbajic/Projects/faiss/ivf_quantizer_leak+0xb5524) (BuildId: 45a0934ffe20c59ae7f414fa8f8845dcec85cc8b)
    #3 0x588a0cf5618a in faiss::index_factory(int, char const*, faiss::MetricType, bool) (/home/jbajic/Projects/faiss/ivf_quantizer_leak+0xb618a) (BuildId: 45a0934ffe20c59ae7f414fa8f8845dcec85cc8b)

SUMMARY: LeakSanitizer: 128 byte(s) leaked in 1 allocation(s).

And it can be reproduced with this example:

#include <faiss/impl/FaissException.h>
#include <faiss/index_factory.h>
#include <cstdio>

int main() {
    try {
        delete faiss::index_factory(8, "IVF4,PQ3");
    } catch (faiss::FaissException const& e) {
        fprintf(stderr, "threw: %s\n", e.what());
    }
}

and run with this: g++ -std=c++17 -g -fsanitize=leak -I. ivf_quantizer_leak.cpp build/faiss/libfaiss.a -llapack -lblas -fopenmp -o ivf_quantizer_leak && ./ivf_quantizer_leak