Add support for constants to fx_glow (#52094)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52094
Pull Request resolved: https://github.com/pytorch/glow/pull/5329
Nested constants are created as placeholders by the graph_splitter used in the partitioner. So we change them back to get_attr nodes before serializing the graph.
Reviewed By: jfix71
Differential Revision: D26375577
fbshipit-source-id: 66631aadd6f5b8826ffa0a1e70176fbcaa7431d5
