Add env-based API for cub::DeviceMergeSort

[gonidelis]

#7968

• Add env-based cub::DeviceMergeSort::SortPairs()
• Add env-based cub::DeviceMergeSort::SortKeys()
• Add env-based cub::DeviceMergeSort::StableSortPairs()
• Add env-based cub::DeviceMergeSort::StableSortKeys()

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#tobeadded

• Add env-based cub::DeviceMergeSort::SortKeysCopy()
• Add env-based cub::DeviceMergeSort::SortPairsCopy()
• Add env-based cub::DeviceMergeSort::StableSortKeysCopy()