add fake fp16 fusions to net transforms (#42927)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42927
added fp16 fusion to net transforms
refactored the transforms as well as glow_transform to get out of opt/custom so that the OSS builds passed
Test Plan: added net runner tests for this
Reviewed By: yinghai
Differential Revision: D23080881
fbshipit-source-id: ee6451811fedfd07c6560c178229854bca29301f
