ns for fx: add weight matching for linear fp16 emulation (#54257)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54257
Makes the NS weight extraction fuction work correctly with
fp16 emulation patterns for linear. We navigate to the
weight correctly, and cast it to `torch.float16` before returning.
Test Plan:
```
python test/test_quantization.py TestFXNumericSuiteCoreAPIs.test_linear_fp16
```
Imported from OSS
Reviewed By: jerryzh168
Differential Revision: D27159370
fbshipit-source-id: 95f555298e3153e4783c64b3d8c83b9d3fdffa12
