Open source fbgemm fp16 pack op (#36791)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36791
This should enable `test_fc_nnpi_fp16.py` test in fakelowp test.
Test Plan:
```
buck test caffe2/caffe2/fb/fbgemm:
```
Reviewed By: hyuen
Differential Revision: D21085221
fbshipit-source-id: 512bca2eea1a4cc5d11129cfe9e65e7a4a0ba1e0
