ns for fx: allow user functions in shadowing (#56301)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56301
Allows usage of user functions in NS shadow APIs. We expose the
i/o mapping to the user APIs, and thread them throughout the code.
Note: the format of the mapping is currently not the best. Saving
improving that for a future PR.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_user_defined_function
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27833189
fbshipit-source-id: dac418e294d1c9b204efbf4071d5cc12a9e784c0
