Tuck the packing logic into Int8FCPackWeight op (#34289)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/34289
Test Plan:
```
buck test caffe2/caffe2/quantization/server:fully_connected_dnnlowp_op_test
```
Reviewed By: csummersea
Differential Revision: D20275538
fbshipit-source-id: 699ca2a145c7c9a50b0fdab7bd68d8557a031ac0
