Don't serialize None values in observer (#32733)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32733
Similar to https://github.com/pytorch/pytorch/pull/32318, we should stop serializing None values since they can't be broadcasted
Test Plan: Imported from OSS
Differential Revision: D19611586
Pulled By: jerryzh168
fbshipit-source-id: 369881de0567ed8eb25bdada892227f49bb5b29d
