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

Serialize a flat tensor file. The resulting file looks like:

Header with
- flatbuffer offset and size
- segment data offset and size

Flatbuffer
Tensor data (in segment)

Differential Revision: [D66374253](https://our.internmc.facebook.com/intern/diff/D66374253/)

[ghstack-poisoned]

Author: lucylq

extension/flat_tensor/serialize/serialize.py

@@ -179,9 +179,9 @@ def __init__(self, config: Optional[FlatTensorConfig] = None) -> None:
         eg. alignment.
         """
         if config is None:
-            self.config = FlatTensorConfig()
+            self.config: FlatTensorConfig = FlatTensorConfig()
         else:
-            self.config = config
+            self.config: FlatTensorConfig = config
 
     def serialize(
         self,