[torch package][easy] Make all the save/load tests use buffers (#98798)
Summary:
Make it a bit easier to run the tests anywhere/avoid skipping the tests by using buffers instead of temporary files.
[Er, still figuring out how the sync tooling works, I'll send this against github once the first diff is landed]
Test Plan: buck2 test
Reviewed By: fluckydog232
Differential Revision: D44818261
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98798
Approved by: https://github.com/ezyang
