[XLA:CPU] Use thread pool to run host kernel functions in parallel.
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[XLA:CPU] Use thread pool to run host kernel functions in parallel.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/tensorflow/pull/66023 from Intel-tensorflow:gaurides/add_back_bf16_sum_mean b9077f91f4c1462b37b4c03bfb5aa8e1d8908933
