TPU related small model.

def imagetransformer_b12l_4h_uncond_dr03_tpu():
  """TPU related small model."""
  hparams = imagetransformer_b12l_4h_b256_uncond_dr03_tpu()
  hparams.learning_rate = 0.2
  hparams.learning_rate_warmup_steps = 4000
  hparams.layer_preprocess_sequence = "none"
  hparams.layer_postprocess_sequence = "dan"
  hparams.layer_prepostprocess_dropout = 0.3
  return hparams