quant: add type annotations on quantization.fx.Quantizer class vars (#48343)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48343
Annotates the 4 class variables on `Quantizer` with real types,
fixing the small things uncovered by this along the way.
Test Plan:
```
mypy torch/quantization/
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: supriyar
Differential Revision: D25136212
fbshipit-source-id: 6ee556c291c395bd8d8765a99f10793ca738086f
