PR #12991: [GPU] Refactor GEMM fusion autotuner.
PR #12991: [GPU] Refactor GEMM fusion autotuner.

Imported from GitHub PR https://github.com/openxla/xla/pull/12991


Copybara import of the project:

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f6e85691b51bdd17fc6167ad8f8cb00d7411fec9 by Ilia Sergachev <isergachev@nvidia.com>:

[GPU] Replace a hash map by a vector of pairs in GEMM fusion autotuner.

This will be needed for multi-host autotuning to split tasks between hosts reliably.

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44682f67eb6e9c521f381a7c3dd93fee95f59366 by Ilia Sergachev <isergachev@nvidia.com>:

[GPU][NFC] Refactor for loop iterators.

Merging this change closes #12991

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/12991 from openxla:gemm_fusion_autotuner_refactor 44682f67eb6e9c521f381a7c3dd93fee95f59366
