[ONNX] Fix aten::cat export when arg include parameters (#105373)
Not all fx.Node are guaranteed to have meta["val"]. 'get_attr' nodes
do not. This PR fixes the callsite checking if meta["val"] is symbol.
Fixes #105370
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105373
Approved by: https://github.com/titaiwangms
