serialize torch.Size object (#20952)
Summary:
fixes https://github.com/pytorch/pytorch/issues/20823
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20952
Differential Revision: D15514274
Pulled By: ailzhang
fbshipit-source-id: 8340a40fadfd06063f7f33b0d99d693e74d5defb
