message: fixed AD of custom ops in Python
diff: @@ -2579,7 +2579,6 @@ def custom(cls, *args, **kwargs):
  if len(diff_vars_out) == 0:
  raise Exception("enoki.custom(): internal error!")
  - print(Type)
  detail = _modules.get(Type.__module__ + ".detail")
  if len(diff_vars_in) > 1:
  @@ -2593,8 +2592,8 @@ def custom(cls, *args, **kwargs):
  detail.ad_add_edge(tmp_out.index_ad(), index)
  detail.ad_add_edge(
  - diff_vars_in[0] if len(diff_vars_in) > 1 else tmp_in.index_ad(),
  - diff_vars_out[0] if len(diff_vars_out) > 1 else tmp_out.index_ad(),
  + diff_vars_in[0] if len(diff_vars_in) == 1 else tmp_in.index_ad(),
  + diff_vars_out[0] if len(diff_vars_out) == 1 else tmp_out.index_ad(),
  inst
  )
  