[quant] Add quant APIs to save/load observer state_dict (#44846)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44846
The save function traverses the model state dict to pick out the observer stats
load function traverse the module hierarchy to load the state dict into module attributes depending on observer type
Test Plan:
python test/test_quantization.py TestQuantizeFx.test_save_observer_state_dict
Imported from OSS
Reviewed By: raghuramank100
Differential Revision: D23746821
fbshipit-source-id: 05c571b62949a2833602d736a81924d77e7ade55
