Add support for NCCL

[shi-eric]

• All five collective communication functions should be supported: https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/api/colls.html
• ncclSend and ncclRecv should also be supported: https://docs.nvidia.com/deeplearning/nccl/user-guide/docs/api/p2p.html
• NCCL can be statically linked or dynamically linked, including from conda-forge (https://anaconda.org/conda-forge/nccl)
• Binary size is an obvious concern, so by default dynamically linked NCCL will be assumed.