Move pooling from utils to training

[RdoubleA]

Context

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

Addresses #1436 .

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.
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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/1455

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.80%. Comparing base (ec21546) to head (54cbac4).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
recipes/ppo_full_finetune_single_device.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1455      +/-   ##
==========================================
- Coverage   70.01%   69.80%   -0.22%     
==========================================
  Files         268      272       +4     
  Lines       12937    13053     +116     
==========================================
+ Hits         9058     9111      +53     
- Misses       3879     3942      +63     

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[SalmanMohammadi]

nit: while you're here I actually think this should belong in rlhf/sequence_processing.py and for classification relevant utils to be refactored once we support classification tasks. not blocking on this, up to you.