* Fix quantile tests running on multi-gpus
* Run some gtests with multiple GPUs
* fix mgpu test naming
* Instruct NCCL to print extra logs
* Allocate extra space in /dev/shm to enable NCCL
* use gtest_skip to skip mgpu tests
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Co-authored-by: Hyunsu Philip Cho <chohyu01@cs.washington.edu>
