fix torch.nn.parallel.scatter_gather.gather to handle NamedTuples and handle moving output to CPU (#51104)
Summary:
Fixes #{[50510](https://github.com/pytorch/pytorch/issues/50510)}
Allows ```torch.nn.parallel.scatter_gather.gather``` to accept a list of NamedTuples as input and returns a NamedTuple whose elements are tensors. I added the author's fix using the ```is_namedtuple``` function.
While testing this fix, I encountered a deprecation warning instructing me to use ```'cpu'``` instead of ```-1``` to move the outputs to the CPU. However, doing this causes an assertion error in the ```_get_device_index``` function. I solved this by handling the CPU case in the affected ```forward``` function.
rohan-varma
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51104
Reviewed By: albanD
Differential Revision: D26395578
Pulled By: rohan-varma
fbshipit-source-id: 6e98c9ce1d9f1725973c18d24a6554c1bceae465
