Refactor: Eagle data loading

[h-guo18]

What does this PR do?

Type of change: ?

Overview:

• Jira ticket: https://jirasw.nvidia.com/browse/OMNIML-2955

• New file: modelopt/torch/utils/plugins/transformers_dataset.py based on: https://github.com/NVIDIA/Model-Optimizer/blob/chenhany/vlm_sft_dataset/modelopt/torch/utils/plugins/trasnformers_datasets.py

Usage

# Add a code snippet demonstrating how to use this

Testing

Tested that training curves after changes (both online&offline) is identical with original branch:

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