Prevent import race condition that leaves torch.package.PackagePickler with unwanted dispatch table entries. (#71025)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71025
TL;DR In come cases:
1) user imports `dill`, which mutates `_Pickler.dispatch`,
2) user imports lib that imports `torch.package`
3) `PackagePickler.dispatch = _Pickler.dispatch.copy()` makes a copy of the mutated table
4) user calls `dill.extend(use_dill=False)` to reset `_Pickler.dispatch`, expecting everything to be okay
5) `PackagePickler` is used to pickle something like `ModuleDict`. `PackagePickler.dispatch` has stale entries to dill pickle functions like `save_module_dict`, which sometimes hard-code calls to `StockPickler.save_global`, which is unaware of torch.package module prefixes.
6) Exception is raised, e.g. `Got unhandled exception Can't pickle <class '<torch_package_2>.caffe2.mylib'>: it's not found as <class '<torch_package_2>.caffe2.mylib'>`
Differential Revision: D33483672
fbshipit-source-id: d7cd2a925bedf27c02524a6a4c3132a262f5c984
