Updated the workflow to upload models to S3 (#5232)

Summary:
The upload should not be all or nothing ([example flow](https://github.com/pytorch/executorch/actions/runs/10783442883)). It should upload exported models to S3 if there is at least one artifact.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5232

Test Plan:
- Android: https://github.com/pytorch/executorch/actions/runs/10800212616
  - iOS: https://github.com/pytorch/executorch/actions/runs/10799346884

Reviewed By: huydhn

Differential Revision: D62459630

Pulled By: guangy10

fbshipit-source-id: cbf6c1c9e030089096d126b91ec10a936030e15b