dbr quant: move model level utils into own file (#68346)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68346
Some utility functions for DBR quant need to be aware
of `AutoQuantizationState`. This PR moves them into their own file, so they
can use the type directly without circular imports, and removes the mypy
ignores which are no longer necessary after this change.
Test Plan:
```
python test/test_quantization.py TestQuantizeDBR
```
Reviewed By: jerryzh168
Differential Revision: D32463763
Pulled By: vkuzo
fbshipit-source-id: e2c367de0d5887c61e6d2c3a73d82f7d76af3de1
