[Clarification] Provision for AMD HIP backend for MXNet

In the process of merging HIP Port of MXNet, we are creating a provision for CUDA and HIP backend for CUB as well. CUB being sub-module for MXNet.
Please refer ticket apache/incubator-mxnet#6257 for disucssion with MXNet team
We are ready with the changes, can we raise a Pull Request, so that the changes can be reviewed and merged to master.