[package] remove save_source_file API (#57340)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57340
This API was only used within our own implementation. I couldn't find
any uses anywhere else. Removing it to reduce our overall surface area,
and also because the semantics are unclear in a world where
serialization is deferred to close() time.
Differential Revision: D28114188
Test Plan: Imported from OSS
Reviewed By: anjali411
Pulled By: suo
fbshipit-source-id: 6da53f20518885c7f4359e00e174f5e911906389
