[p2c2] Add support for Int8FCPackWeight in model transformation
Summary:
In order to enable FC int8 quantization in P2C2, we are trying to run the caffe2 op Int8FCPackWeight in the model transformation pipeline.
The net is being generated from the python side, and passed back into C++ and run here: https://fburl.com/diffusion/3zt1mp03, with these dependencies included: https://fburl.com/diffusion/rdjtdtcf
However, when the net is executed, it errors out with:
```
Cannot create operator of type 'Int8FCPackWeight' on the device 'CPU'
```
This diff attempts to fix this issue.
Test Plan:
To reproduce, just this test without
```
buck test //aiplatform/modelstore/transformation/tests:pyper_to_caffe2_dispatcher_test
```
Reviewed By: jspark1105
Differential Revision: D25965167
fbshipit-source-id: a7414669abb8731177c14e8792de58f400970732
