Fix MPI test usage

Only display usage from rank 0 and exit instead of continuing (and seg fault).

test/mpi/mpi_test.cu

@@ -47,7 +47,9 @@ int main(int argc, char *argv[]) {
   MPI_Comm_rank(MPI_COMM_WORLD, &rank);
 
   if (argc < size) {
+    if (rank == 0)
       printf("Usage : %s <GPU list per rank>\n", argv[0]);
+    exit(1);
   }
 
   int gpu = atoi(argv[rank+1]);