[Feature Request] How to directly define a model that includes a qgemm node？

### Describe the feature request

In this issue (https://github.com/microsoft/onnxruntime/issues/19503), we found that sometimes it's not possible to directly convert gemm to qgemm.

Moreover, on this page (https://onnx.ai/onnx/operators/onnx__MatMulInteger.html) I found that QGemm is a custom operator of onnxruntime, which means it's also not possible to directly create a QGemm node using onnx.helper.make_node.

Therefore, I would like to consult on how to directly define a model that includes a qgemm node, rather than obtaining it through conversion?

### Describe scenario use case

Sometimes it's not possible to directly convert gemm to QGemm, and I want to try QGemm instead of QLinearMatMul.

Start by reviewing the related issue 19503 and the ONNX MatMulInteger operator documentation linked in the report. Then inspect the onnx.helper.make_node entry point and ONNX Runtime's QGemm handling. Done means establishing whether a model can directly define a QGemm node and documenting the supported method or limitation.

Written by the indexing model from the issue text.