ns for fx: enable shadowing int8 to int8 (#56205)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56205
Allows for int8 modules to shadow int8 modules. This is useful when
comparing quantized models with different qconfigs.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_int8_shadows_int8
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27807405
fbshipit-source-id: 10c3bc7ab9bb1e6808aa1af23a34c7cf380465fd
