fx quant: add types to observed_module.py (#49607)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49607
Readability
Test Plan:
```
mypy torch/quantization
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D25643895
fbshipit-source-id: b4b8741b07ac4827c3bacd2084df81fbfdd0c2d5
