[XLA:GPU] Fix compute time calculation in the performance model.
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[XLA:GPU] Fix compute time calculation in the performance model.

The compute time should be calculated based on the number of active cores, which is the minimum of the number of blocks and the number of cores on the device.

Originally suggested in https://github.com/openxla/xla/pull/12208.

Co-authored-by: lingzhi98 <lingzhi.zhou@intel.com>
