Enable batch mode for SAS cross encoders (#1987)

* enable batch mode for sas cross encoders

* fix mypy

* comment on top_1 values added

haystack/nodes/evaluator/evaluator.py

@@ -388,24 +388,31 @@ def semantic_answer_similarity(predictions: List[List[str]],
     # Compute similarities
     top_1_sas = []
     top_k_sas = []
+    lengths: List[Tuple[int,int]] = []
 
     # Based on Modelstring we can load either Bi-Encoders or Cross Encoders.
     # Similarity computation changes for both approaches
     if cross_encoder_used:
-        model = CrossEncoder(sas_model_name_or_path)
-        for preds, labels in zip (predictions,gold_labels):
-            # TODO add efficient batch mode: put all texts and labels into grid and extract scores afterwards
-            grid = []
+        model = CrossEncoder(sas_model_name_or_path)        
+        grid = []
+        for preds, labels in zip (predictions,gold_labels):          
             for p in preds:
                 for l in labels:
                     grid.append((p,l))
-            scores = model.predict(grid)
-            top_1_sas.append(np.max(scores[:len(labels)]))
-            top_k_sas.append(np.max(scores))
+            lengths.append((len(preds), len(labels)))
+        scores = model.predict(grid)
+
+        current_position = 0
+        for len_p, len_l in lengths:
+            scores_window = scores[current_position:current_position+len_p*len_l]
+            # Per predicted doc there are len_l entries comparing it to all len_l labels.
+            # So to only consider the first doc we have to take the first len_l entries
+            top_1_sas.append(np.max(scores_window[:len_l]))
+            top_k_sas.append(np.max(scores_window))
+            current_position += len_p*len_l
     else:
         # For Bi-encoders we can flatten predictions and labels into one list
         model = SentenceTransformer(sas_model_name_or_path)
-        lengths: List[Tuple[int,int]] = []
         all_texts: List[str] = []
         for p, l in zip(predictions, gold_labels):                                  # type: ignore
             # TODO potentially exclude (near) exact matches from computations
@@ -417,7 +424,7 @@ def semantic_answer_similarity(predictions: List[List[str]],
 
         # then select which embeddings will be used for similarity computations
         current_position = 0
-        for i, (len_p, len_l) in enumerate(lengths):
+        for len_p, len_l in lengths:
             pred_embeddings = embeddings[current_position:current_position + len_p, :]
             current_position += len_p
             label_embeddings = embeddings[current_position:current_position + len_l, :]