fix rag retriever save pretrained (#7399)

tests/test_retrieval_rag.py

@@ -168,6 +168,11 @@ class RagRetrieverTest(TestCase):
         self.assertEqual(doc_dicts[1]["id"][0], "0")  # max inner product is reached with first doc
         self.assertListEqual(doc_ids.tolist(), [[1], [0]])
 
+    def test_save_and_from_pretrained(self):
+        retriever = self.get_dummy_hf_index_retriever()
+        with tempfile.TemporaryDirectory() as tmp_dirname:
+            retriever.save_pretrained(tmp_dirname)
+
     def test_legacy_index_retriever_retrieve(self):
         n_docs = 1
         retriever = self.get_dummy_legacy_index_retriever()