dbr quant: stop calling eager quant convert (#70247)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70247
Stops calling the eager mode quantization `convert` function
from DBR quant convert, and instead implements the module swaps
manually. This will make it easier to support quantization types
other than static int8 in future PRs.
Test Plan:
```
python test/test_quantization.py -k DBR
```
Reviewed By: jerryzh168
Differential Revision: D33255924
Pulled By: vkuzo
fbshipit-source-id: afdfd61d71833d987bb38aa4d8c3d214f900c03e
