[ONNX] Remove 'diagnose_step' (#99944)
`ThreadFlowLocation`, a.k.a 'step', cannot fully be visualized by `SARIF vscode extension` today.
Discarding `diagnose_step` such that we don't end up creating diagnostics that record things there.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99944
Approved by: https://github.com/justinchuby
