add onnx dependency to the ci

.github/workflows/pull_request.yml

@@ -112,8 +112,10 @@ jobs:
       - name: Install dependencies
         run: |
           uv pip install --system --no-deps .
-          uv pip install --system pytest
+          # onnx is required for onnx export tests
+          # we don't install all dev dependencies here for speed
           uv pip install --system -r requirements.txt
+          uv pip install --system pytest onnx
 
       - name: Initialize submodules
         run: git submodule update --init --recursive