Add sinusoids of different frequencies as layer (vertical) timing signal.

  Args:
    channels: dimension of the timing signal
    layer: layer num
    num_layers: total number of layers

  Returns:
    a Tensor of timing signals [1, 1, channels].

def get_layer_timing_signal_sinusoid_1d(channels, layer, num_layers):
  """Add sinusoids of different frequencies as layer (vertical) timing signal.

  Args:
    channels: dimension of the timing signal
    layer: layer num
    num_layers: total number of layers

  Returns:
    a Tensor of timing signals [1, 1, channels].
  """

  signal = get_timing_signal_1d(num_layers, channels)
  layer_signal = tf.expand_dims(signal[:, layer, :], axis=1)

  return layer_signal