Initial implementation of NNPI Int8FC op
Test Plan:
```
buck test mode/no-gpu glow/fb/test/numerics:test_fc_nnpi_int8nnpi -- --print-passing-detail
```
Reviewed By: hyuen
Differential Revision: D20450490
fbshipit-source-id: c4811cdc994548b6e319d57115434dfc199e07c2
