[caffe2] add cost inference for FusedFakeQuantFC and FusedFakeQuantFCGradient (#44840)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44840
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44762
Move CostInferenceForFCGradient to fc_inference.cc/h to be used in multiple .cc files.
Test Plan: CI
Reviewed By: qizzzh
Differential Revision: D23714877
fbshipit-source-id: d27f33e270a93b0e053f2af592dc4a24e35526cd
