Remove "ft-" prefix from checkpoint shards.

[EugenHotaj]

Fixes #2345.

Context

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Changelog

What are the changes made in this PR?

• As titled

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[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2354

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✅ No Failures

As of commit aedf98c with merge base a8aeb13 ():
💚 Looks good so far! There are no failures yet. 💚

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[felipemello1]

merging when ci is green

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 23.71%. Comparing base (e6b9064) to head (aedf98c).
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2354      +/-   ##
==========================================
- Coverage   23.72%   23.71%   -0.01%     
==========================================
  Files         361      361              
  Lines       21492    21497       +5     
==========================================
+ Hits         5098     5099       +1     
- Misses      16394    16398       +4     

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