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 so that any issues and incompatibilities can be detected and resolved before the ONNX release.

Please implement CPU kernels for new and updated ONNX ops. A list of new and updated ops can be found at ONNX-Release-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.