[export] Serialize cond submodules (#107818)
Cond submodules only return a single tensor, which was not supported by the serializer. Since the serializer assumes that the graph always returns a list -- this is true for the toplevel graph from dynamo, but not true for the subgraphs.
Differential Revision: [D48622687](https://our.internmc.facebook.com/intern/diff/D48622687)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107818
Approved by: https://github.com/avikchaudhuri
