fix example input generation for dynamic shapes (#4207)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4207

Dynamic input shapes minimum bound occaisonally generates 0, which is not a valid input shape. This change fixes that bug

Reviewed By: kirklandsign

Differential Revision: D59617219

fbshipit-source-id: 9c6c8bc136dcacf0bc6d80835d31892aba7e90a7