Properly remove eos_token in llama3 tokenizer if requested by user

[joecummings]

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 #1474

Changelog

• Adds a test to check if we actually strip eos_token if requested by user
• Pass in 'None' for truncate if add_eos=False
• $$$

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.
Example of docstring:

torchtune/torchtune/modules/vision_transformer.py

Line 285 in 6a7951f

Examples:

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/1477

• 📄 Preview Python docs built from this PR

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✅ No Failures

As of commit 1e0501e with merge base 71be8ad ():
💚 Looks good so far! There are no failures yet. 💚

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 11.11111% with 8 lines in your changes missing coverage. Please review.

Project coverage is 27.01%. Comparing base (71be8ad) to head (1e0501e).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...s/torchtune/models/llama3/test_llama3_tokenizer.py	14.28%	6 Missing ⚠️
torchtune/models/llama3/_tokenizer.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1477       +/-   ##
===========================================
- Coverage   72.25%   27.01%   -45.25%     
===========================================
  Files         274      274               
  Lines       13278    13306       +28     
===========================================
- Hits         9594     3594     -6000     
- Misses       3684     9712     +6028     

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