Deprecate SimpoLoss

[SalmanMohammadi]

Context

What is the purpose of this PR? Is it to

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

Closes #2062. We could link to an example repo in a follow up.

Changelog

Added a deprecation message. To be removed in 0.6.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2063

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✅ No Failures

As of commit ef23321 with merge base f3d8d3c ():
💚 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 65.60%. Comparing base (f3d8d3c) to head (0140dc5).

Additional details and impacted files

@@            Coverage Diff             @@
##            main    #2063       +/-   ##
==========================================
+ Coverage   7.35%   65.60%   +58.24%     
==========================================
  Files        277      323       +46     
  Lines      15422    17963     +2541     
==========================================
+ Hits        1135    11785    +10650     
+ Misses     14287     6178     -8109     

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[SalmanMohammadi]

The deprecated wrapper is messing with the isinstance check in the recipe - should we remove this loss? Or warn in the recipe (bad)?

[SalmanMohammadi]

I've removed from the recipe. If the user tries to use this loss in the recipe, it will display the deprecation warning and error out (in a probably unhelpful way).