Enhance error message for MPI unavailability. (#36781)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36781
Mention that you need to to build PyTorch from source to enable MPI.
Additional context:
https://discuss.pytorch.org/t/distributed-pytorch-with-mpi/77106.
ghstack-source-id: 102341246
Test Plan: waitforbuildbot
Differential Revision: D21082009
fbshipit-source-id: 3a3286349e71322726a341dfc743b5978c7d9a56
