ONNX docs missing info about how to remove custom domains

[Jerry-Master]

📚 The doc issue

In the docs about exporting to onnx here there is not a mention of how to remove the functions. The use of aten operators defined as functions creates a problem when converting to tensorrt. When visualizing with netron the functions are composed of simpler official ai.onnx operators which have support for tensorrt but not the custom exported aten operators. There should be a way to save the models without using functions and custom operators and just export the raw operators even if that means more repetitions, but it would make models exportable to tensorrt.

Suggest a potential alternative/fix

No response

cc @svekars @brycebortree