[ONNX] Fix UnsupportedFxNodesAnalysis after onnx dispatcher changes (#105156)
Simplifies the logic to not depend on info within the exception raised. Due to changes
in onnx dispatcher, the diagnostic within exception raised is now different, which broke
this pass in retrieving the unsupported fx node kind. Adds proper unittest.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105156
Approved by: https://github.com/thiagocrepaldi
