Revamp recipe tutorials

[ananthsub]

What does this PR do ?

The current recipe examples are not so useful to users seeing the repo for the first time. these were added initially to demonstrate various ways of launching scripts with nemo-run. as a result, they primarily emphasize the YAML config overrides without providing a basic quickstart/intro.

This PR updates the existing llama3 recipe examples in a sequence of scripts to demonstrate how to:

1. use the recipe configuration in plain python
2. extend to using yaml config overrides for users who desire this interface
3. launch through nemo-run for single node (torchrun) and multi-node (slurm)
4. offer both pretrain + finetune scripts
5. offer a template sbatch script for users who prefer not to use the nemo-run launcher
6. a README to walk users through these scripts

these examples are still intended for demonstration and onboarding purposes. we shoudl not have the expectation that every single model supported in the repo will have corresponding examples like this. instead, we will offer more generic training scripts under scripts/ to use out of the box with other recipes (#1390)

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• Add specific line by line info of high level changes in this PR.

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[ananthsub]

/ok to test 5107a71