[C++ API] Expose AnyValue and AnyModuleHolder classes (#33026)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33026
This PR contains necessary changes to prepare for https://github.com/pytorch/pytorch/pull/33027. It exposes the following classes to public:
1. `torch::nn::AnyValue`, because if the user has optional arguments in their module's forward method, they must also use the `FORWARD_HAS_DEFAULT_ARGS` macro and pass in the default values for those optional arguments wrapped by `torch::nn::AnyValue`.
2. `torch::nn::AnyModuleHolder`, because `torch::nn::Module` needs to declare it as a friend class for it to be able to access `torch::nn::Module`'s protected methods such as `_forward_has_default_args` / `_forward_num_required_args` / `_forward_populate_default_args`.
Test Plan: Imported from OSS
Differential Revision: D19777814
Pulled By: yf225
fbshipit-source-id: 1c9d5aa24f0689154752c426a83ee98f64c9d02f
