So that the tensor's lifetime management is the same as the management built for the NCCL, pre and post kernels.
Also so that on visualizers, they show up in the NCCL stream line. Otherwise if they show up in the compute line, user may get confused (my code does not have these kernels).
The check is thus moved after the point where we depend NCCL stream from the last compute kernel.
Also moved declaration of `checkForNan` from Utils.hpp to NCCLUtils.hpp, and renamed Utils.cu to NCCLUtils.cu.
Differential Revision: [D61957573](https://our.internmc.facebook.com/intern/diff/D61957573)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/134300
Approved by: https://github.com/shuqiangzhang, https://github.com/wconstab
