GPT-OSS Flash Attention and memory-efficient attention via Native PyTorch SDPA

[markrogersjr]

What does this PR do?

Adds support for native PyTorch SDPA for GPT-OSS, including Flash Attention and memory-efficient attention.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: gpt_oss