docs: clarify extended graph optimization targets

[zcxGGmu]

Summary

• Clarify that extended graph optimizations run after graph partitioning.
• Note that offline optimized models created with extended optimizations should use the same execution providers and target hardware as inference.

Fixes #15529

Test Plan

• python focused check for the new extended-optimization hardware/EP note
• git diff --check
• diff secret/conflict scan

[azure-pipelines]

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