ns for fx: fix issue with shadowing nodes of unknown dtype
Summary:
In https://github.com/pytorch/pytorch/pull/61687, a couple of FX Numeric Suite
tests were disabled.
This PR reenables one of these tests. We update the dtype inference logic
of NS to always return a specific type instead of sometimes returning
"fp32 or int8". When the type cannot be deduced by the current logic,
we do not shadow the node.
As a better version of dtype inference becomes available in FX Graph Mode Quantization,
we could migrate this code to use it.
Future PRs in the stack will unbreak other things to enable NS for FX to
work on torchvision again.
Test plan:
```
python test/test_quantization.py -k NumericSuite
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75471
Approved by: https://github.com/jerryzh168
