Update NCCL Version to 2.5.6-2
Update NCCL Version to 2.5.6-2

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=288829866

GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker,GoogleCloudPlatform/PerfKitBenchmarker