Revert "nested instantiation"

[felipemello1]

Reverts #2519

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/2535

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As of commit fc18a65 with merge base 8dadbaa ():
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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.55%. Comparing base (f3587e5) to head (fc18a65).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
torchtune/config/_instantiate.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2535      +/-   ##
==========================================
- Coverage   66.97%   64.55%   -2.42%     
==========================================
  Files         375      375              
  Lines       22295    22259      -36     
==========================================
- Hits        14932    14370     -562     
- Misses       7363     7889     +526     

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