Move portable/util.py into extension/export_util (#4141)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4141

`portable/util.py` is used by multiple example models including llm models. Since we are moving llm models' export libraries into `extension/llm/export`, it makes sense to move `portable/util.py`
into extension as well.

This change does that.

It moves the test into `examples/models` which seems to be a better place for these unit tests since they test the export flow for different models and also rely on some code under `examples/models` like
`EagerModelFactory`.

It also fixes the import hack inside `executorch/examples/models/llama2/builder.py`.
ghstack-source-id: 233035427

Reviewed By: larryliu0820

Differential Revision: D59306348

fbshipit-source-id: cbfbf0db2861cfac27f81cef186fb00b2dbe034d