intel mpi tuning on dodrio

on dodrio intel mpi 2020 and later, we are advised to set following
```
export FI_PROVIDER=mlx
export I_MPI_FABRICS=shm:ofi
export I_MPI_SHM=clx_avx2
```

and sometimes
```
export I_MPI_OFI_EXPERIMENTAL=1
```

not sure what they mean, and how they are different from defaults