ANN warnings/tests updates (rapidsai#4101)

With the update to FAISS 1.7, the [previously observed issue with IVFPQ](rapidsai#3318) has disappeared. However some other issues were recently identified in ANN methods. This PR updates the relevant warnings and pytests accordingly.

Authors:
  - Victor Lafargue (https://github.com/viclafargue)

Approvers:
  - Dante Gama Dessavre (https://github.com/dantegd)

URL: rapidsai#4101

python/cuml/neighbors/nearest_neighbors.pyx

@@ -276,9 +276,10 @@ class NearestNeighbors(Base,
     Notes
     -----
 
-    Warning: IVFPQ might be unstable in this version of cuML.
-    This is due to a known issue in the FAISS release that this
-    cuML version is linked to. (see FAISS issue #1421)
+    Warning: Approximate Nearest Neighbor methods might be unstable
+    in this version of cuML. This is due to a known issue in
+    the FAISS release that this cuML version is linked to.
+    (see cuML issue #4020)
 
     For an additional example see `the NearestNeighbors notebook
     <https://github.com/rapidsai/cuml/blob/branch-0.15/notebooks/nearest_neighbors_demo.ipynb>`_.
@@ -349,11 +350,11 @@ class NearestNeighbors(Base,
         cdef handle_t* handle_ = <handle_t*><uintptr_t> self.handle.getHandle()
         cdef knnIndexParam* algo_params = <knnIndexParam*> 0
         if self.algorithm in ['ivfflat', 'ivfpq', 'ivfsq']:
-            if self.algorithm == 'ivfpq':
-                warnings.warn("\nWarning: IVFPQ might be unstable in this "
-                              "version of cuML. This is due to a known issue "
-                              "in the FAISS release that this cuML version "
-                              "is linked to. (see FAISS issue #1421)")
+            warnings.warn("\nWarning: Approximate Nearest Neighbor methods "
+                          "might be unstable in this version of cuML. "
+                          "This is due to a known issue in the FAISS "
+                          "release that this cuML version is linked to. "
+                          "(see cuML issue #4020)")
 
             if not is_dense(X):
                 raise ValueError("Approximate Nearest Neigbors methods "

python/cuml/test/test_nearest_neighbors.py

@@ -160,12 +160,6 @@ def test_self_neighboring(datatype, metric_p, nrows):
                           ("ivfsq", "numpy")])
 def test_neighborhood_predictions(nrows, ncols, n_neighbors, n_clusters,
                                   datatype, algo):
-    if algo == "ivfpq":
-        pytest.xfail("Warning: IVFPQ might be unstable in this "
-                     "version of cuML. This is due to a known issue "
-                     "in the FAISS release that this cuML version "
-                     "is linked to. (see FAISS issue #1421)")
-
     if not has_scipy():
         pytest.skip('Skipping test_neighborhood_predictions because ' +
                     'Scipy is missing')
@@ -222,19 +216,13 @@ def test_ivfflat_pred(nrows, ncols, n_neighbors, nlist):
 
 @pytest.mark.parametrize("nlist", [8])
 @pytest.mark.parametrize("M", [16, 32])
-@pytest.mark.parametrize("n_bits", [2, 4])
+@pytest.mark.parametrize("n_bits", [4, 6])
 @pytest.mark.parametrize("usePrecomputedTables", [False, True])
 @pytest.mark.parametrize("nrows", [4000])
 @pytest.mark.parametrize("ncols", [128, 512])
 @pytest.mark.parametrize("n_neighbors", [8])
 def test_ivfpq_pred(nrows, ncols, n_neighbors,
                     nlist, M, n_bits, usePrecomputedTables):
-
-    pytest.xfail("Warning: IVFPQ might be unstable in this "
-                 "version of cuML. This is due to a known issue "
-                 "in the FAISS release that this cuML version "
-                 "is linked to. (see FAISS issue #1421)")
-
     algo_params = {
         'nlist': nlist,
         'nprobe': int(nlist * 0.2),