[export] Add exportable attention and kv cache

[larryliu0820]

Summary: As titled.

Test Plan: Rely on new unit tests

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Context

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• add a new feature
• fix a bug
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Changelog

What are the changes made in this PR?

• Adding attention.py in torchtune/modules/_export/
• Adding kv_cache.py in torchtune/modules/_export/
• Adding test_attention.py in tests/torchtune/modules/_export/

Test plan

Please make sure to do each of the following if applicable to your PR. If you're unsure 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)
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• run recipe tests via pytest tests -m integration_test
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• I did not change any public API
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[pytorch-bot]

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[ebsmothers]

I don't think we want to change the default behavior of our unit test workflow here. Can we figure out a way to decouple the export CI a little more?

[larryliu0820]

Can you say a bit more? Are you ok with a separate CI job running these tests with torch nightly installed?

[larryliu0820]

Added Export CI workflow.

[pbontrager]

Thanks for putting up this first commit. I'm not reviewing the modules themselves but the integration with unit tests as this is the first time we're doing this. I left some comments related to this.

[ebsmothers]

OK just discussed with @pbontrager. Let's do the following:

1. Create a workflow export.yaml as you've already done. Configure it to run under the following conditions:
   a) any PR touching the path torchtune/modules/_export (you can use path filters to do this)
   b) nightly

The workflow should run only on tests in the _export folder, something like the command pytest tests/torchtune/modules/_export. Then you can also separately control PyTorch nightlies etc without having to worry about impacting the setup of other test workflows. By running on every PR touching the _export folder, you'll know immediately if you break anything on your PRs. And the nightly runs will ensure we get quick signal on whether other changes in torchtune break anything in the _export folder.

2. Modify other workflows to exclude torchtune/modules/_export. Specifically I think the files gpu_test.yaml and unit_test.yaml will need to change, and you will need to modify pytest tests -> pytest tests --ignore tests/torchtune/modules/_export

[jackzhxng]

Should we also copy over the README?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.22%. Comparing base (89f935f) to head (6e910f1).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2049      +/-   ##
==========================================
+ Coverage   67.69%   68.22%   +0.53%     
==========================================
  Files         323      323              
  Lines       17823    17961     +138     
==========================================
+ Hits        12065    12254     +189     
+ Misses       5758     5707      -51     

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