Fix failing TiledKNNTest unittest

The TiledKNNTest test was faiiling - and it seems to be because
the matrix::select_k code isn't guaranteed to return elements in
sorted order. The test was expecting outputs to be sorted, and was
failing because of it. This change fixes the test to sort the
outputs before comparing.

Closes rapidsai#1526

cpp/test/neighbors/knn_utils.cuh

@@ -46,7 +46,8 @@ testing::AssertionResult devArrMatchKnnPair(const T* expected_idx,
                                             size_t rows,
                                             size_t cols,
                                             const DistT eps,
-                                            cudaStream_t stream = 0)
+                                            cudaStream_t stream = 0,
+                                            bool sort_inputs = false)
 {
   size_t size = rows * cols;
   std::unique_ptr<T[]> exp_idx_h(new T[size]);
@@ -57,16 +58,30 @@ testing::AssertionResult devArrMatchKnnPair(const T* expected_idx,
   raft::update_host<T>(act_idx_h.get(), actual_idx, size, stream);
   raft::update_host<DistT>(exp_dist_h.get(), expected_dist, size, stream);
   raft::update_host<DistT>(act_dist_h.get(), actual_dist, size, stream);
+
   RAFT_CUDA_TRY(cudaStreamSynchronize(stream));
   for (size_t i(0); i < rows; ++i) {
+    std::vector<std::pair<DistT, T>> actual;
+    std::vector<std::pair<DistT, T>> expected;
     for (size_t j(0); j < cols; ++j) {
       auto idx      = i * cols + j;  // row major assumption!
       auto exp_idx  = exp_idx_h.get()[idx];
       auto act_idx  = act_idx_h.get()[idx];
       auto exp_dist = exp_dist_h.get()[idx];
       auto act_dist = act_dist_h.get()[idx];
-      idx_dist_pair exp_kvp(exp_idx, exp_dist, raft::CompareApprox<DistT>(eps));
-      idx_dist_pair act_kvp(act_idx, act_dist, raft::CompareApprox<DistT>(eps));
+      actual.push_back(std::make_pair(act_dist, act_idx));
+      expected.push_back(std::make_pair(exp_dist, exp_idx));
+    }
+    if (sort_inputs) {
+      // inputs could be unsorted here, sort for comparison
+      std::sort(actual.begin(), actual.end());
+      std::sort(expected.begin(), expected.end());
+    }
+    for (size_t j(0); j < cols; ++j) {
+      auto act = actual[j];
+      auto exp = expected[j];
+      idx_dist_pair exp_kvp(exp.second, exp.first, raft::CompareApprox<DistT>(eps));
+      idx_dist_pair act_kvp(act.second, act.first, raft::CompareApprox<DistT>(eps));
       if (!(exp_kvp == act_kvp)) {
         return testing::AssertionFailure()
                << "actual=" << act_kvp.idx << "," << act_kvp.dist << "!="

cpp/test/neighbors/tiled_knn.cu

@@ -178,7 +178,8 @@ class TiledKNNTest : public ::testing::TestWithParam<TiledKNNInputs> {
                                                        num_queries,
                                                        k_,
                                                        float(0.001),
-                                                       stream_));
+                                                       stream_,
+                                                       true));
   }
 
   void SetUp() override