Replace tokens instead of masking.

def denoise_z15():
  """Replace tokens instead of masking."""
  hparams = xmoe2_dense_0()
  hparams.decoder_type = "denoising"
  hparams.noising_spec_train = {"type": "random_zipfian", "prob": 0.15}
  hparams.noising_use_eval_during_train = 0.25
  return hparams