[quant][graphmode][fx] Add observation_type.py (#67063)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67063
Adding ObservationType Enum for `backend_config_dict`
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D31849078
fbshipit-source-id: e9e7225d564b51fa9454f7f087dd134152c069a0
