[ONNX] Fix node attributes when namespace is aten (#84211)
When `g.at` is used, the previous clean up in #83136 mistakenly removed the behavior that sets `aten=True` in `_add_attribute`. This PR brings the behavior back.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84211
Approved by: https://github.com/thiagocrepaldi, https://github.com/BowenBao
