Replace `constraints` with `dynamic_shapes` in executorch test (#1608)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1608

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

Reviewed By: angelayi

Differential Revision: D52789848

fbshipit-source-id: d71ed7941374a7936876e67acb9ad01b4c9f6231