[xnnpack][lite-int][4/n] introduce serialization to delegate (#87908)
We introduced the serializer we created in the previous diff to our XNNGraph builder, the purpose of this is to serialize parts of the graph as we build this. At the end, we are able to finish and serialize the xnngraph into a std::string for use when we forward this along to on-device runtime.
The next diff will rebuild the xnngraph from the serialization we introduce here, so testing the serialization of the graph will be done in the next diff
Differential Revision: [D39335580](https://our.internmc.facebook.com/intern/diff/D39335580/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D39335580/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87908
Approved by: https://github.com/digantdesai
