Refactor serialize.py (#9579)

Pull Request resolved: https://github.com/pytorch/executorch/pull/9124

Refactor `_extract_tensors` out of serialize.py, simplify before
serializing named_data ghstack-source-id: 273798696

Differential Revision:
[D70752429](https://our.internmc.facebook.com/intern/diff/D70752429/)

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