Update Llama3 Fine-Tuning Notebook

[roclark]

What does this PR do ?

Update the Llama3-8B PubMeQA fine-tuning example notebook for a seamless experience.

Collection: NLP

Changelog

• Fix an issue where using a base model with an older Transformer Engine version would cause a crash indicating missing model weight files.
• Download the model directly from NGC in the notebook for a full end-to-end example.

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• New Feature
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[ericharper]

LGTM. Thanks!