Update base for 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