separate rgb embeddings.

def imagetransformer_sep_channels_8l_8h():
  """separate rgb embeddings."""
  hparams = imagetransformer_base()
  hparams.num_heads = 8
  hparams.batch_size = 1
  hparams.attention_key_channels = hparams.attention_value_channels = 0
  hparams.hidden_size = 512
  hparams.filter_size = 512
  hparams.num_hidden_layers = 8
  hparams.sampling_method = "random"
  return hparams