Fix cached state bug in ToOutVarPass (#1333)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1333

The Executorch pass `ToOutVarPass` had a bug that only occurred when calling
`to_executorch` on two different models: the missing out vars from model A would
be accidentally remembered and thrown again for model B incorrectly.

While this probably requires a more general fix to the PassRegistry and to the
ExecutorchBackendConfig to use a factory instead of a singleton, for now this is easy
to fix for this one issue by not using an instance, and instead keep it local to each `call`.
Other passes might want to check their `ensures` implementation.

Reviewed By: larryliu0820

Differential Revision: D51771381

fbshipit-source-id: b25f57c5743723143bbb436c9dfb4f2daf330014