ci: pin torch/transformers in export-fixture; onnx in runtime dev deps

transformers 5.15 breaks T5 tracing ('multiple values for use_cache') —
the same regression seen on Modal; CI was installing unpinned latest.
The runtime tests build their tiny-graph zips with the onnx package.

Author: Ronald Tse

runtime/pyproject.toml

@@ -19,7 +19,7 @@ dependencies = [
 ]
 
 [project.optional-dependencies]
-dev = ["pytest>=8.0"]
+dev = ["pytest>=8.0", "onnx>=1.16"]
 
 [tool.setuptools.packages.find]
 where = ["src"]