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: D20794765
Pulled By: yf225
fbshipit-source-id: fad623e5d505b7cfcd76a8c5264f18b7a0a3298c
