Big parameters for Evolved Transformer model on TPU.

def evolved_transformer_big_tpu():
  """Big parameters for Evolved Transformer model on TPU."""
  hparams = add_evolved_transformer_hparams(transformer.transformer_big_tpu())
  hparams.learning_rate_constant = 1 / hparams.learning_rate_warmup_steps ** 0.5
  hparams.learning_rate_schedule = (
      "constant*single_cycle_cos_decay")
  return hparams