Minor fix of the histogram observer in FBL eval flows (#17118)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17118
Fix the bug in quantization eval workflow; Add mul_nets option in histogram observer pybind
Reviewed By: yinghai
Differential Revision: D14085321
fbshipit-source-id: 08e3153148522ebc9512a57144d9a8ad154bb6f8
