The way onnxruntime handle attribute

[lixiaoquan]

In https://github.com/onnx/onnx/blob/main/docs/Operators.md

some attributes are (Optinal), which means it doesn't have to be defined. But in onnxruntime, some attributes which are not Optional can be absent and onnxruntime will use default value. Is it on purpose? Should onnxruntime enforces the attribute exsitence if it is not Optional?

The way onnxruntime handles the attributes does make onnxruntime itself more friendly, but it makes definations in Operators.md ambiguous. Many people think the model is OK as long as onnxruntime can run it, even if the model doesn't conform the Operators.md accurately.