Fix seg faults in CAGRA C++ unit tests (facebookresearch#3552)

Summary:
The issue was that `uniform_int_distribution` generates numbers in the range `[0, ntotal]` and not `[0, ntotal)`, which was an oversight on my part. This PR also attempts to reduce the tolerance for `copyTo` tests as we have seen those fail intermittently.

cc ramilbakhshyiev mdouze cjnolet

Pull Request resolved: facebookresearch#3552

Reviewed By: junjieqi

Differential Revision: D59097786

Pulled By: ramilbakhshyiev

fbshipit-source-id: 9dac4367e25c6c219b116ed172089a2fa2a39c4f

Author: divyegala

faiss/IndexHNSW.cpp

@@ -948,7 +948,7 @@ void IndexHNSWCagra::search(
 
             std::random_device rd;
             std::mt19937 gen(rd());
-            std::uniform_int_distribution<idx_t> distrib(0, this->ntotal);
+            std::uniform_int_distribution<idx_t> distrib(0, this->ntotal - 1);
 
             for (idx_t j = 0; j < num_base_level_search_entrypoints; j++) {
                 auto idx = distrib(gen);

faiss/gpu/test/TestGpuIndexCagra.cu

@@ -182,7 +182,7 @@ void queryTest(faiss::MetricType metric, double expected_recall) {
                 recall_score.view(),
                 test_dis_mds_opt,
                 ref_dis_mds_opt);
-        ASSERT_TRUE(*recall_score.data_handle() > expected_recall);
+        ASSERT_GT(*recall_score.data_handle(), expected_recall);
     }
 }
 
@@ -330,7 +330,7 @@ void copyToTest(
                 recall_score.view(),
                 copy_ref_dis_mds_opt,
                 ref_dis_mds_opt);
-        ASSERT_TRUE(*recall_score.data_handle() > expected_recall);
+        ASSERT_GT(*recall_score.data_handle(), expected_recall);
     }
 }
 
@@ -452,7 +452,7 @@ void copyFromTest(faiss::MetricType metric, double expected_recall) {
                 recall_score.view(),
                 copy_test_dis_mds_opt,
                 test_dis_mds_opt);
-        ASSERT_TRUE(*recall_score.data_handle() > expected_recall);
+        ASSERT_GT(*recall_score.data_handle(), expected_recall);
     }
 }