…acebookresearch#3150)

Summary:
Enables avx512 optimized code (AVX512 subsets F, CD, VL, DQ and BW, which are available for Intel Skylake+ and all AMD Zen4).
Also, introduces `FAISS_OPT_LEVEL` environment variable. Set it to `AVX2`, `AVX512` or empty to pick the appropriate x86_64 instruction set.

Compiled via the following
```
cmake -B build -DCMAKE_BUILD_TYPE=Release -DFAISS_ENABLE_GPU=OFF -DFAISS_OPT_LEVEL=avx512 -DBUILD_TESTING=ON .
make -C build -j 8 faiss_test
make -C build -j 8 swigfaiss
make -C build -j 8 swigfaiss_avx2
make -C build -j 8 swigfaiss_avx512
cd build/faiss/python
python3 setup.py build
python3 setup.py install --force
```

Now, running the following script `1.py`
```
import logging
logging.basicConfig(level=logging.DEBUG)

import faiss
```
produces the following:
```
root@6179abeef23c:~/faiss# LOGLEVEL=DEBUG FAISS_OPT_LEVEL= python3 1.py
DEBUG:faiss.loader:Using  as an instruction set.
INFO:faiss.loader:Loading faiss.
INFO:faiss.loader:Successfully loaded faiss.
root@6179abeef23c:~/faiss# LOGLEVEL=DEBUG FAISS_OPT_LEVEL=AVX2 python3 1.py
DEBUG:faiss.loader:Using AVX2 as an instruction set.
INFO:faiss.loader:Loading faiss with AVX2 support.
INFO:faiss.loader:Successfully loaded faiss with AVX2 support.
root@6179abeef23c:~/faiss# LOGLEVEL=DEBUG FAISS_OPT_LEVEL=AVX512 python3 1.py
DEBUG:faiss.loader:Using AVX512 as an instruction set.
INFO:faiss.loader:Loading faiss with AVX512 support.
INFO:faiss.loader:Successfully loaded faiss with AVX512 support.
root@6179abeef23c:~/faiss# LOGLEVEL=DEBUG python3 1.py
DEBUG:faiss.loader:Environment variable FAISS_OPT_LEVEL is not set, so let's pick the instruction set according to the current CPU
INFO:faiss.loader:Loading faiss with AVX512 support.
INFO:faiss.loader:Successfully loaded faiss with AVX512 support.
```

Pull Request resolved: facebookresearch#3150

Reviewed By: algoriddle

Differential Revision: D51701077

Pulled By: mdouze

fbshipit-source-id: 4db05a287e763ff1ce1f676df7f7402532bf1e9e