Register ONNX exporter under PT2 logging (#105989)
As first step of adopting PT2 logging for ONNX exporter.
Also adds `torch/_logging` to `.github/merge_rules.yaml` for ONNX exporter for easier follow ups.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105989
Approved by: https://github.com/abock, https://github.com/ezyang
