Fix circular deps in loading (#26758)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26758
This PR changes the order in which we import classes and functions so
that is is no longer necessary for them to defined in order in a file,
or for there to be proper import statements in the exported file.
Actually importing a function/class now is driven by the need to resolve
the entity during unpickling, type resolution, or value resolution.
While this should allow significant simplification to the code that
serializes classes, this work has not been done yet in order to avoid
inevitable forward compat issues in the transition period.
Notes:
* Individual functions have been replaced with a SourceImporter object
that exposes a resolveType method. This method loads the type if
it has not been loaded yet, potentially parsing (but not loading)
the file it exists in if that file hasn't been parsed yet.
* Some legacy functionality needed to be added as a method to this object
since the old format still used some of this logic for class resolution.
Test Plan: Imported from OSS
Differential Revision: D17558989
Pulled By: zdevito
fbshipit-source-id: 7eae3470bcbd388c4de463e3462d527776ed46c6
