ns for fx: add ability for fp16 model to shadow fp32 model (#73785)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73785
The conversion from fp32 to fp16 is easily defined, we just did not
have it in NS code yet. This PR adds it.This is needed for some customer models.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_fp16_shadows_fp32
```
Reviewed By: jerryzh168
Differential Revision: D34642873
Pulled By: vkuzo
fbshipit-source-id: 9df505b1ea3f3d3cdb3a5f2409ef3a66f40b7eff
(cherry picked from commit 679cd8a5e24b1cfd7f871dcba3ce8a90de980556)
