Re-organizing collation utils

[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 (please add here)

closes #1261, ref #1249

Changelog

This PR:

• Renames our current collation utility which is used in our SFT recipes to padded_collate_sft, to indicate that it is task-specific.
• Changes our current left-padding collation utilities to a generic left_pad_sequence_function.
• Introduces a generic padded_collate, which can be flexibly configured with a choice of base padding function to suit tasks not covered by padded_collate_sft. This is demonstrated through the PPO recipe currently, but the tests for the function also show how it could be used for e.g. classification tasks.

Test plan

Please make sure to do each of the following if applicable to your PR. (If you're not sure 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.

• 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/1463

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✅ No Failures

As of commit 17f9899 with merge base 0bdd308 ():
💚 Looks good so far! There are no failures yet. 💚

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.09677% with 12 lines in your changes missing coverage. Please review.

Project coverage is 69.97%. Comparing base (0bdd308) to head (41fd7e3).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
recipes/eleuther_eval.py	0.00%	2 Missing ⚠️
recipes/ppo_full_finetune_single_device.py	0.00%	2 Missing ⚠️
recipes/full_finetune_distributed.py	0.00%	1 Missing ⚠️
recipes/full_finetune_single_device.py	0.00%	1 Missing ⚠️
recipes/lora_dpo_distributed.py	0.00%	1 Missing ⚠️
recipes/lora_dpo_single_device.py	0.00%	1 Missing ⚠️
recipes/lora_finetune_distributed.py	0.00%	1 Missing ⚠️
recipes/lora_finetune_single_device.py	0.00%	1 Missing ⚠️
recipes/qat_distributed.py	0.00%	1 Missing ⚠️
torchtune/data/_collate.py	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1463      +/-   ##
==========================================
+ Coverage   69.80%   69.97%   +0.17%     
==========================================
  Files         272      270       -2     
  Lines       13053    13102      +49     
==========================================
+ Hits         9111     9168      +57     
+ Misses       3942     3934       -8     

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

beautiful