Support None's When Calculating Emitted ExecutionPlan Outputs (#2243)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2243

Previously Support for outputting int/float was added in D53256808 for Seamless

This extends on this by dropping None Outputs (Which can be outputted by NanoGPT)

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Reviewed By: JacobSzwejbka, Gasoonjia

Differential Revision: D54328663

fbshipit-source-id: 8aa3c4f85772735c01d0fe0cfa52d6251af93dbc