NNAPI: Add runtime flexible shapes & return shapes (#70334)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70334
* Use 0 for load time flexible shapes
* -1 for runtime flexible shapes
* NNAPI needs return shapes for flexible outputs
Test Plan: Tested via upcoming ops
Reviewed By: dreiss
Differential Revision: D33237922
fbshipit-source-id: 50afdd8e3c6401dfb79b4bc09513c9882a09e5d5
