Feat (brevitas_examples/llm): remove dependencies from optimum-amd

[Giuseppe5]

Reason for this PR

Remove circular dependencies between brevitas and optimum-amd

Changes Made in this PR

Brevitas now implements all the functions required for the LLM entrypoint.

Testing Summary

Risk Highlight

• This PR includes code from another work (please detail).
• This PR contains API-breaking changes.
• This PR depends on work in another PR (please provide links/details).
• This PR introduces new dependencies (please detail).
• There are coverage gaps not covered by tests.
• Documentation updates required in subsequent PR.

Checklist

• Code comments added to any hard-to-understand areas, if applicable.
• Changes generate no new warnings.
• Updated any relevant tests, if applicable.
• No conflicts with destination dev branch.
• I reviewed my own code changes.
• Initial CI/CD passing.
• 1+ reviews given, and any review issues addressed and approved.
• Post-review full CI/CD passing.

[nickfraser]

Please also remove optimum-amd from requirements-llm.txt & from noxfile.py as well.

[nickfraser]

Remove single unused import. I think the rest are required.

[nickfraser]

Just a few more references then LGTM!

src/brevitas_examples/llm/README.md:- optimum-amd (install from main)
tests/brevitas_examples/test_llm.py:# LLM example depends on optimum-amd, which requires PyTorch>=2.2