ns for fx: create subgraph type (#54253)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54253
Creates an `NSSubgraph` type for representing a subgraph instance,
and modifies the NS code to use it. This will enable us to add
more information to the subgraph instance definition without
having to change all the callsites.
Test Plan:
```
mypy torch/quantization
python test/test_quantization.py TestFXGraphMatcher
python test/test_quantization.py TestFXNumericSuiteCoreAPIs
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27158198
fbshipit-source-id: 548785dd90144e2da256c23af990620c778e7cfe
