Add benchmark commit override input

[yhl-amd]

Summary

• Add an optional atom_commit input to the ATOM benchmark workflow for manual runs.
• Use the provided commit SHA for all repository checkout steps, falling back to the workflow ref when it is empty.

Test plan

• Parsed .github/workflows/atom-benchmark.yaml locally with PyYAML.
• Not run: full benchmark workflow, since it requires GPU runners and manual model selection.

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