fix int8 FC (#42691)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42691
fix quantization of FC bias to match nnpi
quantize biases to fp16
Test Plan: improved the unit test to have input tensors in fp32
Reviewed By: tracelogfb
Differential Revision: D22941521
fbshipit-source-id: 00afb70610f8a149110344d52595c39e3fc988ab
