Nccl reduce scatter, all gather (#2727)

* Added reduce scatter and all gather for nccl

* fix unused import, delete unused file

* small fix

* deleted useless condition

* fixed comments

* fix bug in eval_gpu, renamed to sum_scatter, fix docs

* final fix docs

* remove and

* Update mlx/distributed/mpi/mpi.cpp

Co-authored-by: Awni Hannun <awni.hannun@gmail.com>

* fix broken set input output

* fixes set output

* typo

* fix typo

* no cpu, no gpu for reduce scatter

---------

Co-authored-by: Awni Hannun <awni.hannun@gmail.com>

mlx/backend/cuda/primitives.cpp

@@ -40,7 +40,6 @@ NO_GPU_MULTI(Eig)
 NO_GPU_MULTI(Eigh)
 
 namespace distributed {
-NO_GPU_MULTI(AllGather)
 NO_GPU_MULTI(Send)
 NO_GPU_MULTI(Recv)
 } // namespace distributed