[BUG] Fix basic tokenizer no special tokens

[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)

Goal: Toggle add_bos and add_eos for Llama2 tokenizer.
Problem: This is not configurable from the Llama2 tokenizer tokenize_messages, which is now the default API.
Root cause: The underlying method used by the Llama2 tokenizer is tokenize_messages_no_special_tokens, which requires you to pass in bos_id and eos_id and always applies them.

Side quest: Turns out the masking for bos_id and eos_id using this function was INCORRECT (it would always apply the mask from the message instead of the proper mask for the special tokens. I fixed this.

Solution: I made the bos_id and eos_id optional for users to pass in. If they passed it in, I apply them. Otherwise, the method will not add the bos/eos tokens.

Changelog

What are the changes made in this PR?

• Make bos_id and eos_id optional. If they are specified, then I do add them. Otherwise, I don't
• Update all corresponding tests
• Update docstring for tokenize_messages_no_special_tokens

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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and a tutorial example

• I did not change any public API
• I have added an example to docs or docstrings

[pytorch-bot]

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[joecummings]

These were WRONG

[ebsmothers]

Great work

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.09%. Comparing base (9a863c8) to head (846002d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1640      +/-   ##
==========================================
- Coverage   70.96%   69.09%   -1.88%     
==========================================
  Files         295      296       +1     
  Lines       15080    15112      +32     
==========================================
- Hits        10701    10441     -260     
- Misses       4379     4671     +292     

Flag	Coverage Δ
	69.09% <100.00%> (-1.88%)	⬇️

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