Bump onnx from 1.20.1 to 1.21.0 in /training

Bumps [onnx](https://github.com/onnx/onnx) from 1.20.1 to 1.21.0.
- [Release notes](https://github.com/onnx/onnx/releases)
- [Changelog](https://github.com/onnx/onnx/blob/main/docs/Changelog-ml.md)
- [Commits](onnx/onnx@v1.20.1...v1.21.0)

---
updated-dependencies:
- dependency-name: onnx
  dependency-version: 1.21.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

training/requirements.txt

@@ -1,5 +1,5 @@
 numpy==1.26.4
-onnx==1.20.1
+onnx==1.21.0
 onnxruntime-gpu==1.23.2
 onnxslim==0.1.85
 python-dotenv==1.0.1