make tests cheaper

Summary:
Many of the additive quantizer tests are recognized as flaky because the tests timeout in non-optimized stress mode.
This is probably because they don't import

https://www.internalfb.com/code/fbsource/fbcode/faiss/tests/common_faiss_tests.py

that sets the number of threads to 4. This diff fixes that and in addition declares the tests as "heavyweight" so that not too many of them are spawned in parallel in stress mode.

https://www.internalfb.com/intern/wiki/TAE/tpx/Timeouts_and_Sharded_Bundled_mode/#degree-of-parallelism

Hopefully it should fix the flaky tests

Reviewed By: alexanderguzhva

Differential Revision: D38111820

fbshipit-source-id: 7dd7c72e7e92b82384a170743cfd5c4aaf9a6960

tests/test_fast_scan.py

@@ -14,6 +14,8 @@ import faiss
 
 from faiss.contrib import datasets
 
+# the tests tend to timeout in stress modes + dev otherwise
+faiss.omp_set_num_threads(4)
 
 class TestCompileOptions(unittest.TestCase):
 

tests/test_fast_scan_ivf.py

@@ -14,6 +14,8 @@ import faiss
 from faiss.contrib import datasets
 from faiss.contrib.inspect_tools import get_invlist
 
+# the tests tend to timeout in stress modes + dev otherwise
+faiss.omp_set_num_threads(4)
 
 class TestLUTQuantization(unittest.TestCase):
 

tests/test_local_search_quantizer.py

@@ -14,6 +14,7 @@ import unittest
 
 from faiss.contrib import datasets
 
+faiss.omp_set_num_threads(4)
 
 sp = faiss.swig_ptr
 

tests/test_residual_quantizer.py

@@ -15,6 +15,7 @@ from faiss.contrib.inspect_tools import get_additive_quantizer_codebooks
 # Reference implementation of encoding with beam search
 ###########################################################
 
+faiss.omp_set_num_threads(4)
 
 def pairwise_distances(a, b):
     anorms = (a ** 2).sum(1)