ns: add compare_weights API with a single model (#92058)
Summary:
Adds a compare weights NS API using a single model.
Note: this is not intended for wide usage, so testing is limited
to specific functions our customers care about. The main reason for adding this
is because existing customers of NS are using the old `compare_weights` API,
and we'd like to move everyone to a single-model API style.
Once all the customers are moved over, we can delete all the old NS code.
Test plan:
```
python test/test_quantization.py -k NShadows.test_extract_weights_linear
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92058
Approved by: https://github.com/jerryzh168
