Remove orphan from cuDNN persistent note (#65160)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/60009.
As the document is properly [included](https://github.com/pytorch/pytorch/blob/master/torch/nn/modules/rnn.py#L799), and [`:orphan:` doesn't need to be used in included documents](https://github.com/sphinx-doc/sphinx/issues/6787#issuecomment-549256840), and no warning is emitted in my local build when removing it, I think it can be removed.
The artifact reported in https://github.com/pytorch/pytorch/issues/60009 can be seen in 3 pages: [torch.nn.RNN](https://pytorch.org/docs/stable/generated/torch.nn.RNN.html#torch.nn.RNN), [torch.nn.LSTM](https://pytorch.org/docs/stable/generated/torch.nn.LSTM.html#torch.nn.LSTM), and [torch.nn.GRU](https://pytorch.org/docs/stable/generated/torch.nn.GRU.html#torch.nn.GRU).
cc ezyang suo
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65160
Reviewed By: bdhirsh
Differential Revision: D31020280
Pulled By: ezyang
fbshipit-source-id: 6c3541e5a856a91cf1ce1d2db4d04f5d13118ee4
