[ONNX] Fix triu/tril export with diagonal input (#86843)
Investigation with @thiagocrepaldi discovered this bug with triu/tril export when
`diagonal` is passed in as input. Previously assumption was made that `diagonal`
is always provided a constant value.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86843
Approved by: https://github.com/thiagocrepaldi, https://github.com/abock
