insert_observers use qconfig_dict (#25069)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25069
This PR changes the API of insert_observers to use qconfig_dict,
full functionality support will come in later PRs
Test Plan:
```
python test/test_quantizer.py
python test/test_jit.py
```
Imported from OSS
Differential Revision: D17001135
fbshipit-source-id: 16df6fa521fcc0c9e268a375be8e1a630e77011a
