add ability to persist memory banks created for faiss (#375)

* init

* add tests

* fix tests'

* more fixes

* add tests

* make the default path more faiss specific

* fix linter

---------

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

llama_stack/providers/impls/meta_reference/memory/tests/test_faiss.py

@@ -0,0 +1,73 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import tempfile
+
+import pytest
+from llama_stack.apis.memory import MemoryBankType, VectorMemoryBankDef
+from llama_stack.providers.impls.meta_reference.memory.config import FaissImplConfig
+
+from llama_stack.providers.impls.meta_reference.memory.faiss import FaissMemoryImpl
+from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
+
+
+class TestFaissMemoryImpl:
+    @pytest.fixture
+    def faiss_impl(self):
+        # Create a temporary SQLite database file
+        temp_db = tempfile.NamedTemporaryFile(suffix=".db", delete=False)
+        config = FaissImplConfig(kvstore=SqliteKVStoreConfig(db_path=temp_db.name))
+        return FaissMemoryImpl(config)
+
+    @pytest.mark.asyncio
+    async def test_initialize(self, faiss_impl):
+        # Test empty initialization
+        await faiss_impl.initialize()
+        assert len(faiss_impl.cache) == 0
+
+        # Test initialization with existing banks
+        bank = VectorMemoryBankDef(
+            identifier="test_bank",
+            type=MemoryBankType.vector.value,
+            embedding_model="all-MiniLM-L6-v2",
+            chunk_size_in_tokens=512,
+            overlap_size_in_tokens=64,
+        )
+
+        # Register a bank and reinitialize to test loading
+        await faiss_impl.register_memory_bank(bank)
+
+        # Create new instance to test initialization with existing data
+        new_impl = FaissMemoryImpl(faiss_impl.config)
+        await new_impl.initialize()
+
+        assert len(new_impl.cache) == 1
+        assert "test_bank" in new_impl.cache
+
+    @pytest.mark.asyncio
+    async def test_register_memory_bank(self, faiss_impl):
+        bank = VectorMemoryBankDef(
+            identifier="test_bank",
+            type=MemoryBankType.vector.value,
+            embedding_model="all-MiniLM-L6-v2",
+            chunk_size_in_tokens=512,
+            overlap_size_in_tokens=64,
+        )
+
+        await faiss_impl.initialize()
+        await faiss_impl.register_memory_bank(bank)
+
+        assert "test_bank" in faiss_impl.cache
+        assert faiss_impl.cache["test_bank"].bank == bank
+
+        # Verify persistence
+        new_impl = FaissMemoryImpl(faiss_impl.config)
+        await new_impl.initialize()
+        assert "test_bank" in new_impl.cache
+
+
+if __name__ == "__main__":
+    pytest.main([__file__])