[ONNX] Expose OnnxRegistry publicly (#106140)
The official move of `OnnxRegistry` to `torch.onnx` allows it to become one of the parameters in `torch.onnx.ExportOption`. By incorporating `OnnxRegistry` in `torch.onnx.ExportOption`, users gain access to various functionalities, including the ability to register custom operators using `register_custom_op`, check whether an operator is supported using `is_registered_op`, and obtain symbolic functions that support specific operators using `get_functions`.
Additionally, `opset_version` is now exclusively available in `torch.onnx.OnnxRegistry` as it is removed from `torch.onnx.ExportOption`. The initialization of the registry with torchlib under the provided opset version ensures that the exporter uses the specified opset version as the primary version for exporting.
These changes encompass scenarios where users can:
1. Register an unsupported ATen operator with a custom implementation using onnx-script.
2. Override an existing symbolic function (onnx invariant).
NOTE: The custom registered function will be prioritized in onnx dispatcher, and if there are multiple custom ones, the one registered the last will be picked.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/106140
Approved by: https://github.com/justinchuby, https://github.com/thiagocrepaldi
