clear predict_net field from PredictorExporterMeta stored in the exporter to save memory (#68485)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68485
In OSS, the only change is that we make the predict_net field of PredictorExporterMeta nullable.
Test Plan: sandcastle, let CI run
Reviewed By: boryiingsu
Differential Revision: D32467138
fbshipit-source-id: 81bd5fca695462f6a186bcfa927073874cc9c26a
