Merge type stubs for torch.nn.parallel (#101528)
Fixes #91648
As explained in the tracking issue, the incomplete type stubs in `torch/nn/parallel` mask `DataParallel` methods relevant for subclassing and also mask type issues present in the code as well.
One notable change here is the addition of [`allow_redefinition = True`](https://mypy.readthedocs.io/en/stable/config_file.html#confval-allow_redefinition) in `mypy.ini`, which allows for a common pattern:
> Allows variables to be redefined with an arbitrary type, as long as the redefinition is in the same block and nesting level as the original definition.
This is added specifically to allow for the type narrowing of `device_ids` in `torch.nn.parallel.data_parallel.data_parallel` from `Sequence[Union[int, torch.device]]` to `Sequence[int]`.
Other than this, there are various renamings and `type: ignore` comments added to bypass errors that arose from the merging.
@ezyang
Pull Request resolved: https://github.com/pytorch/pytorch/pull/101528
Approved by: https://github.com/ezyang
