[export] improve FakeTensor serialization (#119531)
Recently we made it possible to serialize ExportedPrograms with fake parameters/buffers/etc.
The serialization regime was kind of whacky; basically we serialized a stub and reassembled the FakeTensor using metadata that we had stashed elsewhere in the Graph state.
This was bad for a few reasons:
- Storing the metadata separately from the actual serialized object caused situations where you could have one but not the other. An example case is if you had a FakeTensor contained inside a TorchBind object—there was no obviously place to store the metadata for this. This actually happens—TensorQueue in fbgemm does this.
- It created an annoying cycle: we had to deserialize the Graph's tensor metadata in order to deserialize (potentially faked) constants, but we need constants in order to deserialize the Graph.
This fixes all that. The basic idea is to patch the reducer function for FakeTensor at serialization time, and serialize a copy of the FakeTensor metadata. We already are policing BC for the TensorMeta schema struct so it's not a net increase in the BC surface.
As a bonus, I fixed a weird bug with torchbind tracing where we were accidentally reinterpreting a torch.ScriptObject as a torch.ScriptModule (which was the root cause of some weird behavior @bahuang was seeing last week).
Differential Revision: [D53601251](https://our.internmc.facebook.com/intern/diff/D53601251/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119531
Approved by: https://github.com/zhxchen17
