refactor: support qwen3 reranker

Author: sigridjineth

backends/candle/src/lib.rs

@@ -597,13 +597,33 @@ impl Backend for CandleBackend {
         let batch_size = batch.len();
 
         let results = self.model.predict(batch).e()?;
-
-        let results = results.to_dtype(DType::F32).e()?.to_vec2().e()?;
+        let results = results.to_dtype(DType::F32).e()?;
 
         let mut predictions =
             HashMap::with_capacity_and_hasher(batch_size, BuildNoHashHasher::default());
-        for (i, r) in results.into_iter().enumerate() {
-            predictions.insert(i, r);
+
+        // Handle both 1D and 2D tensors
+        match results.dims() {
+            [_n] => {
+                // 1D tensor (e.g., ListwiseReranker returns a vector of scores)
+                let scores = results.to_vec1::<f32>().e()?;
+                for (i, score) in scores.into_iter().enumerate() {
+                    predictions.insert(i, vec![score]);
+                }
+            }
+            [_, _] => {
+                // 2D tensor (normal case)
+                let results = results.to_vec2().e()?;
+                for (i, r) in results.into_iter().enumerate() {
+                    predictions.insert(i, r);
+                }
+            }
+            dims => {
+                return Err(BackendError::Inference(format!(
+                    "Unexpected tensor shape: {:?}",
+                    dims
+                )));
+            }
         }
 
         Ok(predictions)

backends/candle/src/models/bert.rs

@@ -597,6 +597,9 @@ impl BertModel {
                 };
                 (pool, None, splade)
             }
+            ModelType::ListwiseReranker => {
+                candle::bail!("`reranker` model type is not supported for Bert")
+            }
         };
 
         let (embeddings, encoder) = match (
@@ -615,7 +618,6 @@ impl BertModel {
                 }
             }
         };
-
         Ok(Self {
             embeddings,
             encoder,
@@ -661,6 +663,9 @@ impl BertModel {
                 };
                 (pool, None, splade)
             }
+            ModelType::ListwiseReranker => {
+                candle::bail!("`reranker` model type is not supported for RoBERTa")
+            }
         };
 
         let (embeddings, encoder) = match (

backends/candle/src/models/distilbert.rs

@@ -462,6 +462,9 @@ impl DistilBertModel {
 
                 (pool, None)
             }
+            ModelType::ListwiseReranker => {
+                candle::bail!("`reranker` model type is not supported for DistilBert")
+            }
         };
 
         let (embeddings, encoder) = match (