big 1d model for conditional image generation.

def imagetransformer_base_12l_8h_big():
  """big 1d model for conditional image generation."""
  hparams = imagetransformer_sep_channels_8l_8h()
  hparams.filter_size = 1024
  hparams.num_decoder_layers = 12
  hparams.batch_size = 1
  hparams.hidden_size = 512
  hparams.learning_rate_warmup_steps = 4000
  hparams.sampling_method = "random"
  hparams.beam_size = 1
  hparams.block_width = 256
  return hparams