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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def xmoe_2d():
 """Two-dimensional hierarchical mixture of 16 experts."""

  hparams = xmoe_top_2()
  hparams.decoder_layers = ["att", "hmoe"] * 4
  hparams.mesh_shape = "b0:2;b1:4"
  hparams.outer_batch_size = 4
  hparams.layout = "outer_batch:b0;inner_batch:b1,expert_x:b1,expert_y:b0"
  hparams.moe_num_experts = [4, 4]
  return hparams