text: Title: Support NCCL for Elastic Horovod
  Body: The documentation says that Elastic Horovod only works with Gloo. But NCCL is the state-of-the-art collective communication library with GPUs and has been widely adopted in distributed data-parallel DNN training. Therefore, I hope Elastic Horovod will support NCCL. BTW, I wonder why you choose Gloo rather than NCCL. 
  
  > Horovod >= 0.20.0 with Gloo support (install Horovod using HOROVOD_WITH_GLOO=1 to ensure it is installed)
  
  