[caffe2/fakelowp] fix bug in ref code (#38331)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38331
C_ref size was wrong
Test Plan: CI
Reviewed By: hyuen
Differential Revision: D21525639
fbshipit-source-id: 59f4709238cdd46bb38f7c534335eb79229f6c7f
