Makes sure a `y` argument is a vliad numpy dataset.

def _check_y(y):
  """
  Makes sure a `y` argument is a vliad numpy dataset.
  """
  if not isinstance(y, np.ndarray):
    raise TypeError("y must be numpy array. Typically y contains "
                    "the entire test set labels. Got " + str(y) + " of type " + str(type(y)))