relax test_ivf_train_2level threshold (facebookresearch#2927)

Summary: Pull Request resolved: facebookresearch#2927

Reviewed By: mlomeli1

Differential Revision: D47017009

fbshipit-source-id: cfa1df4b9632b085d3a61b56d8617bebd7e5aad6

Author: algoriddle

tests/test_contrib.py

@@ -521,7 +521,7 @@ def test_ivf_train_2level(self):
 
         # normally 47 / 200 differences
         ndiff = (Iref != Inew).sum()
-        self.assertLess(ndiff, 50)
+        self.assertLess(ndiff, 51)
 
 
 class TestBigBatchSearch(unittest.TestCase):