Set default similarity for Cohere model to cosine

Cohere embeddings are expected to be normalized to unit vectors, but due to floating point precision issues,
our check ({@link DenseVectorFieldMapper#isNotUnitVector(float)}) often fails.
This change fixes this bug by setting the default similarity for newly created Cohere inference endpoint to cosine.

Closes #122878

Author: jimczi

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/services/cohere/CohereService.java

@@ -15,6 +15,7 @@
 import org.elasticsearch.common.util.LazyInitializable;
 import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.TimeValue;
+import org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper;
 import org.elasticsearch.inference.ChunkedInference;
 import org.elasticsearch.inference.ChunkingSettings;
 import org.elasticsearch.inference.InferenceServiceConfiguration;
@@ -349,19 +350,18 @@ public Model updateModelWithEmbeddingDetails(Model model, int embeddingSize) {
     }
 
     /**
-     * Return the default similarity measure for the embedding type.
-     * Cohere embeddings are normalized to unit vectors therefor Dot
-     * Product similarity can be used and is the default for all Cohere
-     * models.
+     * Returns the default similarity measure for the embedding type.
+     * Cohere embeddings are expected to be normalized to unit vectors, but due to floating point precision issues,
+     * our check ({@link DenseVectorFieldMapper#isNotUnitVector(float)}) often fails.
+     * Therefore, we use cosine similarity to ensure compatibility.
      *
-     * @return The default similarity.
+     * @return The default similarity measure.
      */
     static SimilarityMeasure defaultSimilarity(CohereEmbeddingType embeddingType) {
         if (embeddingType == CohereEmbeddingType.BIT || embeddingType == CohereEmbeddingType.BINARY) {
             return SimilarityMeasure.L2_NORM;
         }
-
-        return SimilarityMeasure.DOT_PRODUCT;
+        return SimilarityMeasure.COSINE;
     }
 
     @Override

x-pack/plugin/inference/src/test/java/org/elasticsearch/xpack/inference/services/cohere/CohereServiceTests.java

@@ -1605,9 +1605,9 @@ public void testDefaultSimilarity_BinaryEmbedding() {
     }
 
     public void testDefaultSimilarity_NotBinaryEmbedding() {
-        assertEquals(SimilarityMeasure.DOT_PRODUCT, CohereService.defaultSimilarity(CohereEmbeddingType.FLOAT));
-        assertEquals(SimilarityMeasure.DOT_PRODUCT, CohereService.defaultSimilarity(CohereEmbeddingType.BYTE));
-        assertEquals(SimilarityMeasure.DOT_PRODUCT, CohereService.defaultSimilarity(CohereEmbeddingType.INT8));
+        assertEquals(SimilarityMeasure.COSINE, CohereService.defaultSimilarity(CohereEmbeddingType.FLOAT));
+        assertEquals(SimilarityMeasure.COSINE, CohereService.defaultSimilarity(CohereEmbeddingType.BYTE));
+        assertEquals(SimilarityMeasure.COSINE, CohereService.defaultSimilarity(CohereEmbeddingType.INT8));
     }
 
     public void testInfer_StreamRequest() throws Exception {