separate rgb embeddings.

def imagetransformer_sep_channels_12l_16h_imagenet_large():
  """separate rgb embeddings."""
  hparams = imagetransformer_sep_channels_8l_8h()
  hparams.num_hidden_layers = 12
  hparams.batch_size = 1
  hparams.filter_size = 2048
  hparams.num_heads = 16
  hparams.learning_rate_warmup_steps = 16000
  hparams.sampling_method = "random"
  hparams.learning_rate = 0.1
  return hparams