[flatbuffer] Move saving storage to the last step. (#78024)
Summary: Move storage saving to last step, because otherwise tensors saved after storage are already saved will not have storage.
Test Plan: Tested by loading the file in `clowder get GLDGLQnKrIsQFg8DAPxq9vg59ZwZbmQwAAAA orig.pt` and converting to flatbuffer and load again
Differential Revision: D36552645
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78024
Approved by: https://github.com/Jack-Khuu
