[executorch][flat_tensor] Serialize flat tensor tests

Pull Request resolved: https://github.com/pytorch/executorch/pull/7269

Introduce _convert_to_flat_tensor, which interprets a flat_tensor blob as a flat_tensor schema.

Use this for more comprehensive testing for flat tensor serialization, and later for deserialization.

ghstack-source-id: 261976100
@exported-using-ghexport

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

Co-authored-by: lucylq <lfq@meta.com>