Improve C++ API autograd and indexing docs (#35777)
Summary:
This PR adds docs for the following components:
1. Tensor autograd APIs (such as `is_leaf` / `backward` / `detach` / `detach_` / `retain_grad` / `grad` / `register_hook` / `remove_hook`)
2. Autograd APIs: `torch::autograd::backward` / `grad` / `Function` / `AutogradContext`, `torch::NoGradGuard` / `torch::AutoGradMode`
3. Tensor indexing
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35777
Differential Revision: D20810616
Pulled By: yf225
fbshipit-source-id: 60526ec0c5b051021901d89bc3b56861c68758e8
