Remove skip building graph check for quantization use case

For quantization indices, we don't have to apply building graph check
since it is already faster, this is now only applied for fp32/16 indices
and where threshold is configured.

Signed-off-by: Vijayan Balasubramanian <[email protected]>

src/main/java/org/opensearch/knn/index/codec/KNN990Codec/NativeEngines990KnnVectorsWriter.java

@@ -104,9 +104,8 @@ public void flush(int maxDoc, final Sorter.DocMap sortMap) throws IOException {
                 field.getVectors()
             );
             final QuantizationState quantizationState = train(field.getFieldInfo(), knnVectorValuesSupplier, totalLiveDocs);
-            // Check only after quantization state writer finish writing its state, since it is required
-            // even if there are no graph files in segment, which will be later used by exact search
-            if (shouldSkipBuildingVectorDataStructure(totalLiveDocs)) {
+            // should skip graph building only for non quantization use case and if threshold is met
+            if (quantizationState == null && shouldSkipBuildingVectorDataStructure(totalLiveDocs)) {
                 log.info(
                     "Skip building vector data structure for field: {}, as liveDoc: {} is less than the threshold {} during flush",
                     fieldInfo.name,
@@ -144,9 +143,8 @@ public void mergeOneField(final FieldInfo fieldInfo, final MergeState mergeState
         }
 
         final QuantizationState quantizationState = train(fieldInfo, knnVectorValuesSupplier, totalLiveDocs);
-        // Check only after quantization state writer finish writing its state, since it is required
-        // even if there are no graph files in segment, which will be later used by exact search
-        if (shouldSkipBuildingVectorDataStructure(totalLiveDocs)) {
+        // should skip graph building only for non quantization use case and if threshold is met
+        if (quantizationState == null && shouldSkipBuildingVectorDataStructure(totalLiveDocs)) {
             log.info(
                 "Skip building vector data structure for field: {}, as liveDoc: {} is less than the threshold {} during merge",
                 fieldInfo.name,