Fix/custom dataset chat template

[mreso]

What does this PR do?

This PR fixes the custom dataset example for tokenizer that add system prompt by default.

Fixes # (issue)
#669

Feature/Issue validation/testing

Please describe the tests that you ran to verify your changes and relevant result summary. Provide instructions so it can be reproduced.
Please also list any relevant details for your test configuration.

• pytest src/tests/datasets/test_custom_dataset.py -s -k test_tokenize_dialog

========================================================================================================================================================================== test session starts ===========================================================================================================================================================================
platform linux -- Python 3.10.14, pytest-8.3.2, pluggy-1.5.0
rootdir: /home/mreso/llama-recipes
configfile: pyproject.toml
plugins: anyio-4.4.0, mock-3.14.0
collected 6 items / 4 deselected / 2 selected

src/tests/datasets/test_custom_dataset.py ..

============================================================================================================================================================================ warnings summary ============================================================================================================================================================================
src/tests/datasets/test_custom_dataset.py::test_tokenize_dialog[meta-llama/Llama-2-7b-hf]
  /home/mreso/llama-recipes/src/llama_recipes/model_checkpointing/checkpoint_handler.py:17: DeprecationWarning: `torch.distributed._shard.checkpoint` will be deprecated, use `torch.distributed.checkpoint` instead
    from torch.distributed._shard.checkpoint import (

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
=============================================================================================================================================================== 2 passed, 4 deselected, 1 warning in 3.10s ===============================================================================================================================================================

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• Did you read the contributor guideline,
  Pull Request section?
• Did you write any new necessary tests?

Thanks for contributing 🎉!

[init27]

Thanks for adding the tests as well!