allow passing dynamic shape information in examples dir (#1471)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1471

Right now nothing under examples can have dynamic shape information. This change correctly forwards it along if the user implements "get_dynamic_shape"

Reviewed By: larryliu0820

Differential Revision: D52301732

fbshipit-source-id: 457df819a63e59bd3cc539cc4038ff02d4205b75