[retrievers][knn] Replace loop appends with list comprehension. (#5529)

# Replace loop appends with list comprehension.

It's much faster, more idiomatic and slightly more readable.

langchain/retrievers/knn.py

@@ -51,13 +51,14 @@ class KNNRetriever(BaseRetriever, BaseModel):
         denominator = np.max(similarities) - np.min(similarities) + 1e-6
         normalized_similarities = (similarities - np.min(similarities)) / denominator
 
-        top_k_results = []
-        for row in sorted_ix[0 : self.k]:
+        top_k_results = [
+            Document(page_content=self.texts[row])
+            for row in sorted_ix[0 : self.k]
             if (
                 self.relevancy_threshold is None
                 or normalized_similarities[row] >= self.relevancy_threshold
-            ):
-                top_k_results.append(Document(page_content=self.texts[row]))
+            )
+        ]
         return top_k_results
 
     async def aget_relevant_documents(self, query: str) -> List[Document]: