[C++ API] Update torch::nn layer docs (#34522)
Summary:
This PR updates C++ API torch::nn layer docs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34522
Test Plan: Imported from GitHub, without a `Test Plan:` line.
Differential Revision: D20380832
Pulled By: yf225
fbshipit-source-id: ee99a838ec05c6ce2a23aa97555707e507d09958
