[export] Add exportable position embedding

[larryliu0820]

Summary:

Add _position_embeddings.py under _export/

Test Plan:

Rely on new unit tests.

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Tasks:

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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)

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Changelog

What are the changes made in this PR?

• torchtune/modules/_export/_position_embeddings.py
• test/torchtune/modules/_export/_position_embeddings.py

Test plan

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• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
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UX

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

[pytorch-bot]

🔗 Helpful Links

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As of commit 1a9bf51 with merge base 1450d61 ():
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