Add support to `Tensor[]?` for structured kernel codegen.
This PR turns the previously introduced `ITensorList` into a more general `IList`
class. It is a container wrapper for arbitrary types (given their appropriate
implementations).
In summary, I have:
- Renamed `ITensorList` (its iterators and macros, for consistency) to `IList`
- Made `IList` a templated function (for an arbitrary type `T`), given that they:
- Specialize `IListTagImpl<T, Tag>`, for all `IListTag`
- Introduced type aliases (for both list and iterator types):
- `at::ITensorList` -> `c10::IList<at::Tensor>`
- `at::IOptTensorRefList` -> `c10::IList<at::OptionalTensorRef>`
- Added support for `Tensor?[]` in the structured codegen
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69606
Approved by: https://github.com/ezyang
