Handle fused scale and bias in fake fp16 layernorm
Summary: Allow passing scale and bias to fake fp16 layernorm.
Test Plan: net_runner. Now matches glow's fused layernorm.
Reviewed By: hyuen
Differential Revision: D22952646
fbshipit-source-id: cf9ad055b14f9d0167016a18a6b6e26449cb4de8
