skip nn.Identity in add_observer
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/23500
Test Plan:
e2e test in quantizing resnext 101
Imported from OSS
Differential Revision: D16550190
Pulled By: jerryzh168
fbshipit-source-id: 6128d7c3419235152b43739fcc5cade34342ba3d
