Fix unaries to export fp16 instead of fp32 when rest of the model export to int8
Summary: Currently accelerators does not have the concept for fp32, it only has understandings of fp16 and int8 in terms of data input. In order to fixe the issue here, we want to make sure unaries are turned into fp16 when we have the int8 exporter turned on.
Reviewed By: kennyhorror
Differential Revision: D17743791
fbshipit-source-id: 7322d23eb12ac3f813b525fc0ddd066f95c8ca85
