Update phi-3-mini to use the export library (#4190)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4190

as title.

Note: exporting model from HF is currently blocked by this issue, https://fburl.com/qpaapajr. We need to apply the following trick to export the model.
```
        model = torch.export._trace._export(
            model,
            example_inputs,
            dynamic_shapes=dynamic_shape,
            strict=False,
            pre_dispatch=False,
        )
```
ghstack-source-id: 233317600
exported-using-ghexport

Reviewed By: larryliu0820

Differential Revision: D59503255

fbshipit-source-id: dca72327c02d6289a295b894d1a8abc344b187cc