Add example script to run NeMo 2.0 llama pretraining with NeMo-Run
I feel like we should hold off on this until we merge the new entrypoint-API in NeMo-Run, see [docs](https://github.com/NVIDIA/NeMo-Run/blob/c284dcc7005ac5a2183dac661173eee46e05df87/examples/entrypoint/README.md). This will simplify this example considerably while adding more flexibility. > I feel like we should hold off on this until we merge the new entrypoint-API in NeMo-Run, see [docs](https://github.com/NVIDIA/NeMo-Run/blob/c284dcc7005ac5a2183dac661173eee46e05df87/examples/entrypoint/README.md). This will simplify this example considerably while adding more flexibility.

@marcromeyn  But what if we don't want the extra flexibility. The aim of this example is to be able to reproduce exactly what we have been doing. Users can then use CLI or the script itself for personalization.  Yeah I think we can create a separate CLI folder to host the CLI examples, but would be nice to have both so the users get a clear picture. It will also help show the benefits via a 1:1 comparison. @ShriyaPalsamudram if the script would only show how to do one thing, so let's say llama3-8b I would agree. But this script offers some form of parameterization using argparse + the `MODEL_SIZE_MAPPING` dict . I believe when we showcase scripts with a form of parameterization we should be showcasing the functionality inside NeMo-Run that is designed for that.
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```python
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```

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