Custom Llama2 export script (#1412)

Summary:
Enable llama export with custom weights (.pth) and config (.json)

Run with `python3 -m examples.models.llama2.export_llama --checkpoint "demo_rand_params.pth" --params "demo_config.json"`

Update documentation to reflect new changes.

Pull Request resolved: https://github.com/pytorch/executorch/pull/1412

Test Plan:
Imported from GitHub, without a `Test Plan:` line.

```
python3 -m examples.models.llama2.export_llama --checkpoint "demo_rand_params.pth" --params "demo_config.json"
```

Reviewed By: larryliu0820

Differential Revision: D52148409

Pulled By: lucylq

fbshipit-source-id: 320584ea884b43f0b76ec308c02470d6553d2f48