Add llama 8b 128k recipe and mixtral 8x64k 64k recipe

[BoxiangW]

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Collection: [Note which collection this PR will affect]

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Additional Information

• Related to # (issue)

To fix the problem, we need to remove the unused import statement. This will clean up the code and remove the unnecessary dependency. Specifically, we will remove the finetune import from the line from nemo.collections.llm.api import finetune, pretrain.