Residual feed-forward layer with normalization at start.

def ResidualFeedForward(feature_depth,
                        feedforward_depth,
                        dropout,
                        mode):
  """Residual feed-forward layer with normalization at start."""
  return layers.Residual(
      layers.LayerNorm(),
      layers.Dense(feedforward_depth),
      layers.Relu(),
      layers.Dropout(rate=dropout, mode=mode),
      layers.Dense(feature_depth),
      layers.Dropout(rate=dropout, mode=mode)
  )