Add system prompt

[RdoubleA]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Enable adding an optional system prompt in our built-in message transforms. Expose this parameter in our dataset builders. Any user dataset that uses our common message transforms can leverage this feature.

Addresses #1328

Test plan

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UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it.
Example of docstring:

torchtune/torchtune/modules/vision_transformer.py

Line 285 in 6a7951f

Examples:

Example in our docs: https://pytorch.org/torchtune/main/tutorials/qat_finetune.html#applying-qat-to-llama3-models

• I did not change any public API;
• I have added an example to docs or docstrings;

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1366

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[joecummings]

Need some way to convey that this will either add or override the given system prompt:

new_system_prompt?

[joecummings]

I think these need to be updated to or at least the documentation stating that this will override any system prompt?

[RdoubleA]

oi, missed these

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.12195% with 2 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@4ae950b). Learn more about missing BASE report.

Files	Patch %	Lines
torchtune/data/_messages.py	90.47%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1366   +/-   ##
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  Coverage        ?   72.69%           
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  Files           ?      271           
  Lines           ?    12805           
  Branches        ?        0           
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  Hits            ?     9308           
  Misses          ?     3497           
  Partials        ?        0           

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