Decorator that recomputes the function on the backwards pass.

  Args:
    fn: a function that takes Tensors (all as positional arguments) and returns
      a tuple of Tensors.

  Returns:
    A wrapped fn that is identical to fn when called, but its activations will
    be discarded and recomputed on the backwards pass (i.e. on a call to
    tf.gradients).

def recompute_grad(fn):
  """Decorator that recomputes the function on the backwards pass.

  Args:
    fn: a function that takes Tensors (all as positional arguments) and returns
      a tuple of Tensors.

  Returns:
    A wrapped fn that is identical to fn when called, but its activations will
    be discarded and recomputed on the backwards pass (i.e. on a call to
    tf.gradients).
  """

  @functools.wraps(fn)
  def wrapped(*args):
    return _recompute_grad(fn, args)

  return wrapped