[PyTorchEdge] Start writing magic to flatbuffer output (#74084)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74084
Now that the schema includes a magic file header string, write it to the flatbuffer data generated by `flatbuffer_serializer`.
ghstack-source-id: 151109277
Test Plan: A later diff in this stack (D34408538) tests that the output data contains the magic header.
Reviewed By: pavithranrao
Differential Revision: D34809318
fbshipit-source-id: edb45d57e56fa4b30675eb9ce6e4e258abfd5417
(cherry picked from commit f5e8a3ff70eba186ac9e7b91739010e55cd6c5a6)
