Update docs feature classifications (#39966)
Summary:
Update the following feature classifications in docs to align with the changes:
1. [High Level Autograd APIs](https://pytorch.org/docs/stable/autograd.html#functional-higher-level-api): Beta (was experimental)
2. [Eager Mode Quantization](https://pytorch.org/docs/stable/quantization.html): Beta (was experimental)
3. [Named Tensors](https://pytorch.org/docs/stable/named_tensor.html): Prototype (was experimental)
4. [TorchScript/RPC](https://pytorch.org/docs/stable/rpc.html#rpc): Prototype (was experimental)
5. [Channels Last Memory Layout](https://pytorch.org/docs/stable/tensor_attributes.html#torch-memory-format): Beta (was experimental)
6. [Custom C++ Classes](https://pytorch.org/docs/stable/cpp_index.html): Beta (was experimental)
7. [Torch.Sparse](https://pytorch.org/docs/stable/sparse.html): Beta (was experimental)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39966
Differential Revision: D22213217
Pulled By: jlin27
fbshipit-source-id: dc49337cbc7026ed8dcac506fc60029dc3add854
