[ONNX] Clean up `onnx_supported_ops` (#79424)
- Hide the module from `torch.onnx` public namespace because it is for internal use
- Remove unused variables
- Fix lint errors
- Reformat
- Create `onnx` folder under docs/scripts and add it to the onnx merge rule
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79424
Approved by: https://github.com/thiagocrepaldi, https://github.com/garymm, https://github.com/kit1980, https://github.com/malfet
