fix faiss serialize and serialize of index (#464)

faiss serialize index returns a np object, that we first need to save to
buffer and then write to sqllite. Since we are using json, we need to
base64 encode the data.

Same in the read path, we base64 decode and read into np array and then
call into deserialize index.

tests:
torchrun $CONDA_PREFIX/bin/pytest -v -s -m "faiss"
llama_stack/providers/tests/memory/test_memory.py

Co-authored-by: Dinesh Yeduguru <dineshyv@fb.com>

llama_stack/providers/inline/memory/faiss/faiss.py

@@ -5,6 +5,7 @@
 # the root directory of this source tree.
 
 import base64
+import io
 import json
 import logging
 
@@ -67,19 +68,20 @@ class FaissIndex(EmbeddingIndex):
                 for k, v in data["chunk_by_index"].items()
             }
 
-            index_bytes = base64.b64decode(data["faiss_index"])
-            self.index = faiss.deserialize_index(index_bytes)
+            buffer = io.BytesIO(base64.b64decode(data["faiss_index"]))
+            self.index = faiss.deserialize_index(np.loadtxt(buffer, dtype=np.uint8))
 
     async def _save_index(self):
         if not self.kvstore or not self.bank_id:
             return
 
-        index_bytes = faiss.serialize_index(self.index)
-
+        np_index = faiss.serialize_index(self.index)
+        buffer = io.BytesIO()
+        np.savetxt(buffer, np_index)
         data = {
             "id_by_index": self.id_by_index,
             "chunk_by_index": {k: v.json() for k, v in self.chunk_by_index.items()},
-            "faiss_index": base64.b64encode(index_bytes).decode(),
+            "faiss_index": base64.b64encode(buffer.getvalue()).decode("utf-8"),
         }
 
         index_key = f"faiss_index:v1::{self.bank_id}"
@@ -188,7 +190,7 @@ class FaissMemoryImpl(Memory, MemoryBanksProtocolPrivate):
     ) -> None:
         index = self.cache.get(bank_id)
         if index is None:
-            raise ValueError(f"Bank {bank_id} not found")
+            raise ValueError(f"Bank {bank_id} not found. found: {self.cache.keys()}")
 
         await index.insert_documents(documents)