[C++ API] Update torch::nn functional docs (#34688)
Summary:
- `torch::nn::functional` functions must provide example for how to use the corresponding functional options
- `torch::nn::functional` functions must link to the corresponding functional options
- remove `TORCH_NN_FUNCTIONAL_USE_MODULE_OPTIONS` macro, and put `torch::nn::functional` options docs inside the functional namespace, right above functional declaration
- `torch::nn::functional` options docs should not link back to torch::nn layers. Instead, they should have links to `torch::nn::functional::xxx`
----
This PR is BC-breaking in the following way:
`TORCH_NN_FUNCTIONAL_USE_MODULE_OPTIONS` macro is removed, and user should explicitly write
```cpp
namespace functional {
using SomeFuncOptions = SomeModuleOptions;
} // namespace functional
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34688
Differential Revision: D20431251
Pulled By: yf225
fbshipit-source-id: 7d4f27dca3aad2a1e523690927d7afb261b9d308
