fx quant: refactor qconfig_dict utils to separate file (#69636)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69636
Moves some of the qconfig_dict utilities away from the FX subdirectory
into the quantization subdirectory. These utilities can be reused with
other workflows.
A future PR will start using these utilities in DBR quant.
Test Plan:
```
python test/test_quantization.py TestQuantizeFx
```
Reviewed By: albanD
Differential Revision: D33003474
Pulled By: vkuzo
fbshipit-source-id: 34417b198681279469e6d7c43ea311180086d883
