Adds message_transform link from SFTDataset docstring to docs

[thomasjpfan]

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)

Fixes #1331

Changelog

What are the changes made in this PR?

• Adds message_transform link from SFTDataset docstring to docs

Test plan

Please make sure to do each of the following if applicable to your PR. If you're unsure about any one of these just ask and we will happily help. We also have a contributing page for some guidance on contributing.

• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
• add unit tests for any new functionality
• update docstrings for any new or updated methods or classes
• run unit tests via pytest tests
• run recipe tests via pytest tests -m integration_test
• manually run any new or modified recipes with sufficient proof of correctness
• include relevant commands and any other artifacts in this summary (pastes of loss curves, eval results, etc.)

UX

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

• 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/2219

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ No Failures

As of commit 4502312 with merge base 5d1866f ():
💚 Looks good so far! There are no failures yet. 💚

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.85%. Comparing base (5d1866f) to head (4502312).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2219   +/-   ##
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  Coverage   23.85%   23.85%           
=======================================
  Files         344      344           
  Lines       20658    20658           
=======================================
  Hits         4928     4928           
  Misses      15730    15730           

☔ View full report in Codecov by Sentry.
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[RdoubleA]

Looks great, and renders correctly. Thanks for the fix!