ns for fx: unify return types of weight and activation APIs (#52779)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52779
1. makes the return type of the weight comparison APIs match the return
type of the activation comparison APIs:
```
# before
{layer_name: {model_name: weight_tensor}}
{layer_name: {model_name: [activation_tensor]}}
# after
{layer_name: {model_name: [weight_tensor]}}
{layer_name: {model_name: [activation_tensor]}}
```
2. makes a type alias for the type, so future changes are easier
Test Plan:
```
mypy torch/quantization
python test/test_quantization.py TestFXNumericSuiteCoreAPIs
```
Imported from OSS
Reviewed By: hx89
Differential Revision: D26652639
fbshipit-source-id: eb1f04d6913cedf88d628f362468875ae9ced928
