Small single parameters.

def mtf_image_transformer_base_single():
  """Small single parameters."""
  hparams = mtf_image_transformer_base()
  hparams.num_decoder_layers = 6
  hparams.filter_size = 256
  hparams.block_length = 128
  hparams.mesh_shape = ""
  hparams.layout = ""
  return hparams