Disable mpi on version mismatch (#1989)

mlx/distributed/mpi/mpi.cpp

@@ -69,6 +69,7 @@ struct MPIWrapper {
     if (version.find("Open MPI") == std::string::npos) {
       std::cerr << "[mpi] MPI found but it does not appear to be Open MPI."
                 << "MLX requires Open MPI but this is " << version << std::endl;
+      libmpi_handle_ = nullptr;
       return;
     }