ns for fx: add fp16 function shadowing (#56311)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56311
Adds functionality for shadowing user functions with fp16 I/O dtype.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_user_defined_function
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27836064
fbshipit-source-id: 37a434a04e2bd2593a892209bbae59f0f1f34319
