Add C++ nn::Identity (#26713)
Summary:
**Summary**:
Adds `torch::nn::Identity` module support for the C++ API.
**Issue**: https://github.com/pytorch/pytorch/issues/25883
**Reviewer**: yf225
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26713
Differential Revision: D17550982
Pulled By: yf225
fbshipit-source-id: f24483846e82d5d276d77a1a0c50884f3bc05112
