Update on "[executorch][flat_tensor] Serialize flat tensor tests"

More comprehensive testing for flat tensor serialization.

Differential Revision: [D67007821](https://our.internmc.facebook.com/intern/diff/D67007821/)

[ghstack-poisoned]

Author: lucylq

extension/flat_tensor/test/test_serialize.py

@@ -8,6 +8,8 @@
 
 import unittest
 
+from typing import List
+
 from executorch.exir._serialize.data_serializer import (
     DataPayload,
     DataSerializer,
@@ -27,8 +29,6 @@
     FlatTensorSerializer,
 )
 
-from typing import List
-
 # Test artifacts.
 TEST_TENSOR_BUFFER: List[bytes] = [b"\x11" * 4, b"\x22" * 32]
 TEST_TENSOR_MAP = {