add net transforms for fusion (#42763)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42763
add the fp16 fusions as net transforms:
-layernorm fused with mul+add
-swish int8
Test Plan: added unit test, ran flows
Reviewed By: yinghai
Differential Revision: D23002043
fbshipit-source-id: f0b13d51d68c240b05d2a237a7fb8273e996328b
