ns for fx: add fp16 function shadowing (#57023)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57023
Adds functionality for shadowing user functions with fp16 I/O dtype.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_user_defined_function
```
Reviewed By: jerryzh168
Differential Revision: D28030092
Pulled By: vkuzo
fbshipit-source-id: 642792398a76bd62593fa439ab14901e9dbdf4f8
