[doc][modules] update to modules documentation

[felipemello1]

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)

Changelog

• Added docstring that were missing;
• Changed some variable names like i, k, v to more meaningful names;
• Updated RST with missing API facing functions and deleted non existing ClassifierTransfomer
• Minor changes in existing docstrings

Test plan

make html - confirmed it rendered well

• 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.)

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1079

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ No Failures

As of commit 539ad06 with merge base dceaf05 ():
💚 Looks good so far! There are no failures yet. 💚

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[joecummings]

Couple of nits, mostly looks good.

[joecummings]

LGTM, thanks!