error out when tag input/output node (#1398)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1398

coreml is working on adding partitioner and they accidently tag output node. The stack trace is not straightforward to read. We can add an assertion early to point user what goes wrong.

Reviewed By: tarun292

Differential Revision: D52060445

fbshipit-source-id: 4f9a3014d6e9914996f1bd116170376589d4edf8