Expose script_if_tracing as public API (#46494)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/45921
`torch.jit._script_if_tracing` is still kept for BC
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46494
Reviewed By: ZolotukhinM
Differential Revision: D24381621
Pulled By: gmagogsfm
fbshipit-source-id: 35d9f2da38c591039ba95cd95ef186e6c7e47586
