ns for fx to backend_config_dict [2/x]: native lowering mappings
Summary:
NS for FX mappings were originally hardcoded, because quantization op
mappings were not easily reusable. Now that we have `backend_config_dict`,
we can start moving NS for FX to use them and delete the hardcoded mappings.
This PR deletes the hardcoded mappings from NS about the lowering step,
and instead reads them from the lowering configs.
Note: for now, there is no way to configure the tool to use lowering
configs from a different lowering pass. That may be added at some
future point, but it's not important now.
Test plan:
```
python test/test_quantization.py -k FXNumericSuite
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76978
Approved by: https://github.com/jerryzh168
