Add mutli-vector index and batch iterator support (#123)

This PR introduces multi-vector indexing and batch iterator
functionality. With this PR, each label can be associated with multiple
vectors, while ensuring that neighbors are retrieved with unique labels
during search.

- Key APIs for `MultiBatchIterator`:
`next`: Retrieve a batch of neighbors, ensuring each neighbor has a
unique label.

- Key APIs for `MultiMutableVamanaIndex`:
`search`: Utilize `MultiBatchIterator` to perform searches.
`add_points`: Allow the addition of points to the index, associating
each with a corresponding label. Points with the same label can be added
without issue.
`delete_entries`: Remove all vectors associated with a specific label
from the index.
`get_distance`: Return the shortest distance among vectors with the
given label and query.
`make_batch_iterator`: Return a `MultiBatchIterator` associated with the
current index.

- Add `make_batch_iterator` to `MutableVamanaIndex` and `VamanaIndex` as
well to enhance functionality.


#TODOs:

- [x] Add more unittests
- [x] Add different types of distance into unittests
- [ ] Documentation
- [ ] (TBD) Disable translator in `MutableVamana` for
`MultiMutableVamanaIndex`
- [ ] Elevate `MultiMutableVamana` at orchestrator/type-ersure level and
eventually to python bindings

Author: dian-lun-lin

include/svs/index/vamana/dynamic_index.h

@@ -44,6 +44,8 @@
 
 namespace svs::index::vamana {
 
+template <typename Index, typename QueryType> class BatchIterator;
+
 /////
 ///// MutableVamanaIndex
 /////
@@ -722,12 +724,13 @@ class MutableVamanaIndex {
     ///   Delete consolidation performs the actual removal of deleted entries from the
     ///   graph.
     ///
-    template <typename T> void delete_entries(const T& ids) {
+    template <typename T> size_t delete_entries(const T& ids) {
         translator_.check_external_exist(ids.begin(), ids.end());
         for (auto i : ids) {
             delete_entry(translator_.get_internal(i));
         }
         translator_.delete_external(ids);
+        return ids.size();
     }
 
     void delete_entry(size_t i) {
@@ -1244,6 +1247,14 @@ class MutableVamanaIndex {
         // Call extension for distance computation
         return extensions::get_distance_ext(data_, distance_, internal_id, query);
     }
+
+    template <typename QueryType>
+    auto make_batch_iterator(
+        std::span<const QueryType> query,
+        size_t extra_search_buffer_capacity = svs::UNSIGNED_INTEGER_PLACEHOLDER
+    ) const {
+        return BatchIterator(*this, query, extra_search_buffer_capacity);
+    }
 };
 
 ///// Deduction Guides.
@@ -1398,4 +1409,4 @@ auto auto_dynamic_assemble(
         std::move(logger)};
 }
 
-} // namespace svs::index::vamana
+} // namespace svs::index::vamana

include/svs/index/vamana/index.h

@@ -48,6 +48,8 @@
 
 namespace svs::index::vamana {
 
+template <typename Index, typename QueryType> class BatchIterator;
+
 struct VamanaIndexParameters {
     // Members
   public:
@@ -887,6 +889,14 @@ class VamanaIndex {
         // Call extension for distance computation
         return extensions::get_distance_ext(data_, distance_, id, query);
     }
+
+    template <typename QueryType>
+    auto make_batch_iterator(
+        std::span<const QueryType> query,
+        size_t extra_search_buffer_capacity = svs::UNSIGNED_INTEGER_PLACEHOLDER
+    ) const {
+        return BatchIterator(*this, query, extra_search_buffer_capacity);
+    }
 };
 
 // Shared documentation for assembly methods.