Add support for outputting constants (#1774)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1774

For Seamless model we ran into a case where we have to either return an int/float or a list of int's/float's from the model. We need to add support for this in the emitter and make sure that the memory planning pass ignores these.

Reviewed By: angelayi

Differential Revision: D53256808

fbshipit-source-id: 32cffb2bd85a8ef3fca7c8632d571277450ceba9