Mixture of experts (16 experts).

def xmoe_top_2():
  """Mixture of experts (16 experts)."""
  hparams = xmoe_dense_4k()
  moe.set_default_moe_hparams(hparams)
  hparams.mesh_shape = "all:8"
  hparams.layout = "batch:all;experts:all"
  return hparams