[package] Use custom persistent_load in PackageImporter (#51595)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51595
Right now `PackageExporter` defines its own `persistent_id` but
`PackageImporter` uses the one defined in `torch.serialization`. I have
some downstream plans to customize this so this PR just splits it out.
Not to fear! I know this introduces some duplication and potential for
different behavior between `torch.save` and `torch.package`, but I have
plans to re-unify them soon.
Test Plan: Imported from OSS
Reviewed By: zdevito
Differential Revision: D26211578
Pulled By: suo
fbshipit-source-id: 48a2ccaefb2525e1498ad68b75c46d9de3d479b7
