Integrate with ONNX 1.21.0 release branch

[Sunny-Anand]

We are releasing ONNX 1.21.0. A release branch is created (https://github.com/onnx/onnx/tree/rel-1.21.0). The planned release date is April 10, 2026. Release candidates are also available from TestPyPI:

pip install -i https://pypi.org/simple/ --pre onnx==1.21.0rc1

It is important to integrate ONNX release branch into ORT ASAP so that any issues and incompatibilities can be detected and resolved before the ONNX release.

Please follow instructions at (https://github.com/microsoft/onnxruntime/blob/master/docs/How_To_Update_ONNX_Dev_Notes.md) to integrate with the ONNX release branch. Please implement CPU kernels for new and updated ONNX ops. A list of new and updated ops can be found at ONNX-1.21.0.

In case a bug in ONNX is detected during integration of ONNX 1.21.0, please open a ONNX Bug Report and tag ONNX Release Manager @Sunny-Anand so that the bug is fixed in the ONNX release branch.

[titaiwangms]

Integrate with ONNX 1.21.0 release branch

Description

As mentioned above, ONNX 1.21.0rc1 has been released. We should try it within ONNXRUNTIME submodule update to see whether there is any blocker.

We should follow step by step in https://github.com/microsoft/onnxruntime/blob/main/docs/How_To_Update_ONNX_Dev_Notes.md to complete this task (rc1 might need a different installation name/commit where you can check with Previous PR References). It's worth mentioning that ONNXRUNTIME support both _deps.txt and vcpkg installation, and we need to take care of both of them.

https://github.com/onnx/onnx/wiki/Logistics-for-ONNX-Release-1.21.0 provides update detail of this new version. Importantly, we need to register/update new opset version for updated op. In this case, BitCast, CumProd.

Make sure the corresponding tests all pass and updated if needed. Also, the requirements.txt files for Python CI are updated.

For what it worth, #26579 includes commits where we can see how it was gradually updated from even 1.20.0rc1 to official 1.20.1 version.

Previous PR References (make sure you go over these PRs. The task is heavily experienced based)

1. Integration with ONNX==1.20.1 #26579
2. Bump onnx to 1.19.1 #26202
3. Integration with ONNX 1.19 #25678
4. Integration with ONNX 1.18.0 #24945
5. Integration with ONNX rel-1.18.0 #24449
6. Integrate onnx 1.17.0 #21897
7. update onnx 1.16.2 #21565
8. update to onnx 1.16.1 #20702
9. Integration with ONNX 1.16.0 #19745
10. use onnx rel-1.15.0, update cgman, cmake/external and requirement hash #18177
11. Update ONNX to 1.15.0rc1 #17914
12. ONNX 1.15 integration #17125