fx quant: fix types on _find_quants (#49616)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49616
Add types to `_find_quants` I/O and fix resulting errors,
needed for an upcoming bug fix.
Test Plan:
```
mypy torch/quantization
python test/test_quantization.py TestQuantizeFx
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25645719
fbshipit-source-id: 4bf788b55fd4fd086c83a4438b9c2df22b9cff49
