Re-add OnDiskGraphIndex::getDimension

Author: michaeljmarshall

jvector-base/src/main/java/io/github/jbellis/jvector/graph/disk/OnDiskGraphIndex.java

@@ -227,6 +227,10 @@ public Set<FeatureId> getFeatureSet() {
         return features.keySet();
     }
 
+    public int getDimension() {
+        return dimension;
+    }
+
     @Override
     public int size(int level) {
         return layerInfo.get(level).size;