Small encoder-decoder model for testing.

def mtf_bitransformer_tiny():
  """Small encoder-decoder model for testing."""
  hparams = mtf_bitransformer_base()
  hparams.batch_size = 2
  hparams.mesh_shape = ""
  hparams.d_model = 128
  hparams.encoder_layers = ["self_att", "drd"] * 2
  hparams.decoder_layers = ["self_att", "enc_att", "drd"] * 2
  hparams.num_heads = 4
  hparams.d_ff = 512
  return hparams