fix colbert rank calculation (#399)

Author: epinzur

libs/colbert/ragstack_colbert/colbert_retriever.py

@@ -246,16 +246,16 @@ async def _fetch_chunk_data(
                 chunk_data_map[chunk] = chunk_data
 
         answers: List[RetrievedChunk] = []
-        rank = 1
-        for chunk in chunks_by_score:
+
+        for idx, chunk in enumerate(chunks_by_score):
             score = chunk_scores[chunk]
             chunk_data = chunk_data_map[chunk]
             answers.append(
                     RetrievedChunk(
                         doc_id=chunk.doc_id,
                         chunk_id=chunk.chunk_id,
                         score=score.item(),  # Ensure score is a scalar if it's a tensor
-                        rank=rank,
+                        rank=idx + 1,
                         data=chunk_data,
                     )
                 )

libs/colbert/tests/integration_tests/test_colbert_embedding_retrieval.py

@@ -96,4 +96,6 @@ def generate_texts(text, chunk_size, overlap_size):
     for chunk in chunks:
         logging.info(f"got {chunk}")
     assert len(chunks) == 5
-    assert len(chunks[0].data.text) > 0
+    assert len(chunks[0].data.text) > 0
+    assert chunks[0].rank == 1
+    assert chunks[1].rank == 2