Support cosine distance for training vectors (#4227)

Summary:

same as title

Differential Revision: D70724590

benchs/bench_fw/descriptors.py

@@ -106,6 +106,8 @@ class DatasetDescriptor:
     # desc_name
     desc_name: Optional[str] = None
 
+    normalize_L2: bool = False
+
     def __hash__(self):
         return hash(self.get_filename())
 

benchs/bench_fw/index.py

@@ -1138,6 +1138,8 @@ class IndexFromFactory(Index):
                 return None, None, ""
             logger.info(f"assemble, train {self.factory}")
             xt = self.io.get_dataset(self.training_vectors)
+            if self.training_vectors.normalize_L2:
+                faiss.normalize_L2(xt)
             _, t, _ = timer("train", lambda: codec.train(xt), once=True)
             t_aggregate += t