dbr quant: make SeenOpInfo a dataclass (#71267)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71267
Refactors `SeenOpInfo` to be a dataclass, to be consistent with
`QTensorInfo`, so we can get real typing. Fixes the type errors. No logic change.
Test Plan:
```
python test/test_quantization.py -k DBR
```
Reviewed By: HDCharles
Differential Revision: D33567129
Pulled By: vkuzo
fbshipit-source-id: 55f89d7a497b6db1fd9956255d964663032a0401
(cherry picked from commit 7fdec92b9cc9ecbc8ca7224cfec5668543cd8cfc)
