def split_3d_array_into_channels(arr):
  """Split 3D array into a list of 2D arrays.

  e.g. given a numpy array of shape (100, 200, 6), return a list of 6 channels,
  each with shape (100, 200).

  Args:
    arr: a 3D numpy array.

  Returns:
    list of 2D numpy arrays.
  """
  return [arr[:, :, i] for i in range(arr.shape[-1])]