PR #13108: [GPU] Shard GEMM fusion autotuning across multiple compilation processes.
PR #13108: [GPU] Shard GEMM fusion autotuning across multiple compilation processes.

Imported from GitHub PR https://github.com/openxla/xla/pull/13108

This solves the problem of compiler processes compiling different programs due to fluctuations of measurements during autotuning and in addition makes some cases faster (depending on the number of fusions and the availability of CPU cores).
Copybara import of the project:

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73a35a3dc21435061f86e53c02418cc69097e57e by Ilia Sergachev <isergachev@nvidia.com>:

[GPU] Shard GEMM fusion autotuning across compiler processes.

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22201078fadf807dbe00b0f4c3db83f9f015999f by Ilia Sergachev <isergachev@nvidia.com>:

[NFC] Refactor CreateGpuClient().

Merging this change closes #13108

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/13108 from openxla:parallel_autotuning 22201078fadf807dbe00b0f4c3db83f9f015999f
