Fix scalar vs non-scalar issue in kd.attr

```
x = kd.obj()
kd.attr(x, kd.slice(['x']), kd.slice([1]))
```

Crashes due to an incorrect assumption that `x.GetObjSchema()` is never an item

PiperOrigin-RevId: 863658648
Change-Id: Iae5d3423e9ee726df4064ff084e1550505224d3d

Author: felbro

koladata/operators/core.cc

@@ -113,9 +113,9 @@ absl::Status UpdateSchemaForConflictDetection(
 absl::Status UpdateSchemaForConflictDetection(const DataSlice& obj,
                                               const DataSlice& attr_names,
                                               DataBagPtr& result_db) {
-  bool object_mode = obj.GetSchemaImpl() == schema::kObject;
-  ASSIGN_OR_RETURN(DataSlice src_schema,
-                   object_mode ? obj.GetObjSchema() : obj.GetSchema());
+  ASSIGN_OR_RETURN(DataSlice src_schema, obj.GetSchemaImpl() == schema::kObject
+                                             ? obj.GetObjSchema()
+                                             : obj.GetSchema());
   DataSlice dst_schema = src_schema.WithBag(result_db);
 
   ASSIGN_OR_RETURN(auto aligned_slices,
@@ -125,9 +125,7 @@ absl::Status UpdateSchemaForConflictDetection(const DataSlice& obj,
       aligned_slices[1].impl<internal::DataSliceImpl>();
 
   absl::Status status = absl::OkStatus();
-  if (object_mode) {
-    DCHECK(!src_schema.is_item());
-    DCHECK(!dst_schema.is_item());
+  if (!src_schema.is_item()) {
     // In this case, src_schema and dst_schema are not DataItems, so we need to
     // iterate over all sub-items.
     DCHECK(aligned_obj.dtype() == arolla::GetQType<internal::ObjectId>());

py/koladata/operators/tests/core_attr_test.py

@@ -96,6 +96,19 @@ def test_attr_update_with_ds_attr(self):
       testing.assert_equal(ds1.updated(db).x.no_bag(), ds([42, 3]))
       testing.assert_equal(ds1.updated(db).y.no_bag(), ds([2, 42]))
 
+  def test_attr_update_with_ds_attr_scalar_obj(self):
+    with self.subTest('object'):
+      ds1 = kd.obj(x=1)
+      db = kd.core.attr(ds1, ds(['x', 'y']), ds([42, 43]))
+      testing.assert_equal(ds1.updated(db).x.no_bag(), ds(42))
+      testing.assert_equal(ds1.updated(db).y.no_bag(), ds(43))
+
+    with self.subTest('entity'):
+      ds1 = kd.new(x=1)
+      db = kd.core.attr(ds1, ds(['x', 'y']), ds([42, 43]))
+      testing.assert_equal(ds1.updated(db).x.no_bag(), ds(42))
+      testing.assert_equal(ds1.updated(db).y.no_bag(), ds(43))
+
   def test_invalid_attr_name(self):
     o = bag().new(x=1)
     with self.assertRaisesRegex(