Fix pickler bug where it would not load if no tensors were saved
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/23263
Test Plan: Imported from OSS
Differential Revision: D16446928
Pulled By: zdevito
fbshipit-source-id: f70f86b28c3901a97b65b4d7654e39dc6e1aab6a
