fix: correct test failures in test_backends

- Remove spurious .T in asymmetric_distance_computation() that
  caused a broadcast shape mismatch (10,100) vs (100,10)
- Fix off-by-one in test_distributed_index: assert shard count == 4
  instead of checking for non-existent shard index 4
- Skip TestGPUIndex when FAISS is not installed instead of raising
  RuntimeError

Signed-off-by: Maxime Kawawa-Beaudan <maxkb@meta.com>
Signed-off-by: Maxime Grenu <maxime.grenu@gmail.com>

Author: cluster2600

python/tests/test_backends.py

@@ -35,6 +35,10 @@ def test_get_backend_info(self):
         assert "selected" in info
 
 
+@pytest.mark.skipif(
+    not detect.FAISS_AVAILABLE,
+    reason="FAISS not installed",
+)
 class TestGPUIndex:
     """Tests for GPU index."""
 
@@ -251,7 +255,7 @@ def test_distributed_index(self):
         vector_ids = [f"v_{i}" for i in range(100)]
 
         index.add(vectors, vector_ids)
-        assert 4 in index._local_indexes
+        assert len(index._local_indexes) == 4
 
     def test_result_merger(self):
         """Test result merging."""

python/zvec/backends/search.py

@@ -33,7 +33,7 @@ def asymmetric_distance_computation(
     distances = np.zeros((n_queries, n_codes), dtype=np.float32)
 
     for i in range(codes.shape[1]):  # m sub-vectors
-        distances += distance_table[:, i, codes[:, i]].T
+        distances += distance_table[:, i, codes[:, i]]
 
     return distances