Add Union type to TorchScript Language Ref (#69514)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69514
Reviewed By: tugsbayasgalan
Differential Revision: D32909371
Pulled By: gmagogsfm
fbshipit-source-id: af1c3040cd59ee913dc576cf8a8c759313f1e07f
