[ONNX] Relax not exist assertion for 'register_pytree_node' (#107245)
To not conflict with potential existing workaround or solution outside of exporter.
Latest huggingface/transformers main (>4.31) patches PyTorch PyTree with support over `ModelOutput` class.
`_PyTreeExtensionContext` is kept to support prior versions of transformers.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107245
Approved by: https://github.com/titaiwangms
ghstack dependencies: #106741, #107158, #107165
