NDEBUG unused-variable fixes - LangevinSampler
NDEBUG unused-variable fixes - LangevinSampler

chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling,chunshen1987/DistributionSampling