Bug fix: Handle missing keys in observer state dict during load (#30357)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30357
Fix issue https://github.com/pytorch/pytorch/issues/29032 in loading from state dict for observers and fake quant.
ghstack-source-id: 94468814
Test Plan: Ensures that load/save of fake quant and observers with missing keys works correctly.
Differential Revision: D18668517
fbshipit-source-id: 0eda6f47c39102e55977fc548b9a03664f123ad7
