Replace `constraints` with `dynamic_shapes` in pye and executorch (#1714)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1714

X-link: https://github.com/pytorch/pytorch/pull/118345

`constraints` argument for `torch.export` has been deprecated in favor of the `dynamic_shapes` argument. This PR updates the use of the deprecated API in `pye` and `executorch`.

Reviewed By: avikchaudhuri

Differential Revision: D53109299

fbshipit-source-id: 353cdf6cb88e362fc49c957ccdc76a533eb3f283