Add support for multiple inputs to out_wrapper and strict dtype checking (#79941)
When a function returns multiple parameters in PyTorch, the `out`
parameter takes a tuple of tensors (see `linalg.svd` for example).
The current implementation in `out_wrapper_multi` modelled this wrong,
as it assumed that it would take a number of different named
parameters.
This PR implements the correct behaviour in `out_wrapper`. As a small
side-effect, we now need to call `@out_wrapper()` when the output is
just one tensor.
This PR also implements an additional optional parameter that checks
whether the dtype of the given `out` is exactly the dtype that the meta
function requires. This is the behaviour that we currently have in
PyTorch, and this check is necessary in eager when we call with these
tensors into external libraries.
We also make the functions with several outputs return a namedtuple,
similar to what we do in PyTorch.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79941
Approved by: https://github.com/mruberry, https://github.com/ezyang
