Fix: prefer .is_none() over .is(py::none()) for pybind11 in caffe2 (#88199)
Follow up to #88051 . I noticed that I missed a few spots in the caffe2 folder. Prefer `.is_none()` over `.is(py::none())` as `.is_none()` is more efficient since it avoid reference counting increments and decrements.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88199
Approved by: https://github.com/albanD, https://github.com/kit1980
