Returns list from list, tuple or ndarray.

def np_to_list(elem):
  """Returns list from list, tuple or ndarray."""
  if isinstance(elem, list):
    return elem
  elif isinstance(elem, tuple):
    return list(elem)
  elif isinstance(elem, np.ndarray):
    return list(elem)
  else:
    raise ValueError(
        'Input elements of a sequence should be either a numpy array, a '
        'python list or tuple. Got {}'.format(type(elem)))