In copy_weights_to_flat_buf_views() explicitly construct tuple (#43244)
Summary:
In some versions of GCC, tuple constructor from initializer list is marked as explicit, which results in the following compilation error:
```
/var/lib/jenkins/workspace/aten/src/ATen/native/cudnn/RNN.cpp: In function 'std::tuple<at::Tensor, std::vector<at::Tensor, std::allocator<at::Tensor> > > at::native::cudnn_rnn::copy_weights_to_flat_buf_views(at::TensorList, int64_t, int64_t, int64_t, int64_t, int64_t, bool, bool, cudnnDataType_t, const c10::TensorOptions&, bool, bool, bool)':
/var/lib/jenkins/workspace/aten/src/ATen/native/cudnn/RNN.cpp:687:35: error: converting to 'std::tuple<at::Tensor, std::vector<at::Tensor, std::allocator<at::Tensor> > >' from initializer list would use explicit constructor 'constexpr std::tuple<_T1, _T2>::tuple(_U1&&, _U2&&) [with _U1 = at::Tensor&; _U2 = std::vector<at::Tensor>&; <template-parameter-2-3> = void; _T1 = at::Tensor; _T2 = std::vector<at::Tensor>]'
return {weight_buf, params_arr};
```
This regression was introduced by https://github.com/pytorch/pytorch/pull/42385
Fixes #{issue number}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43244
Reviewed By: pbelevich
Differential Revision: D23205656
Pulled By: malfet
fbshipit-source-id: 51470386ad95290c7c99d733fc1fe655aa27d009
