Refactor torch.onnx documentation (#108379)
* Distinguish both TorchScript-based exporter (`torch.onnx.export`) and the TorchDynamo-based exporter (`torch.onnx.dynamo_export`) exporters
* Merge ONNX diagnostics page with the exporter page
* Add initial version of a quick overview on the new exporter
* Updates `torch.compiler.html` with the right page for the ONNX Runtime backend for `torch.compile`
* Renamed doc files to clearly identify files belonging to the legacy and newer onnx exporters
Fixes #108274
https://docs-preview.pytorch.org/pytorch/pytorch/108379/index.html
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108379
Approved by: https://github.com/justinchuby, https://github.com/wschin, https://github.com/malfet
