dbr quant: simplify relatedness logic (#68374)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68374
Cleans up the relatedness logic in DBR quant. For now, this is still
duplicated with NS. A future PR should unify these mappings.
Test Plan:
```
python test/test_quantization.py TestQuantizeDBR
```
Reviewed By: jerryzh168
Differential Revision: D32463750
Pulled By: vkuzo
fbshipit-source-id: 90c2f5e79b86b1b595bd52650305bad88212ed49
