message: ek.custom: accumulate gradients instead overwriting them
diff: @@ -2164,7 +2164,7 @@ class CustomOp:
  return _ek.grad(self.output)
  def set_grad_out(self, value):
  - _ek.set_grad(self.output, value)
  + _ek.accum_grad(self.output, value)
  def grad_in(self, name):
  if name not in self.inputs:
  @@ -2174,7 +2174,7 @@ class CustomOp:
  def set_grad_in(self, name, value):
  if name not in self.inputs:
  raise Exception('Could not find input argument named \"%s\"!' % name)
  - _ek.set_grad(self.inputs[name], value)
  + _ek.accum_grad(self.inputs[name], value)
  def custom(cls, *args, **kwargs):
  