[PyTorch Mobile] Don't use __ROOT__ for inference only ops
Summary:
`__ROOT__` ops are only used in full-jit. To make size compact, disable using it in inference. Since FL is still in fill-jit, keep it for training only.
It saves -17 KB for fbios.
TODO: when FL is migrated to lite_trainer, remove `__ROOT__` to save size in training too.
Test Plan: CI
Reviewed By: dhruvbird
Differential Revision: D24686838
fbshipit-source-id: 15214cebb9d8defa3fdac3aa0d73884b352aa753
