update phi-3-mini readme doc (#4377)

Summary:
Update phi-3-mini readme to reflect latest changes.

It also fallbacks to use `torch.export._trace._export` due to issue https://github.com/pytorch/pytorch/issues/128394

Pull Request resolved: https://github.com/pytorch/executorch/pull/4377

Reviewed By: kirklandsign

Differential Revision: D60130144

Pulled By: helunwencser

fbshipit-source-id: 1d8d9a3791b877f43ad9312b4e704d5ca6d7f69e