Clean up `IterType::GatherScatter`

[naoyam]

It was used to indicate domain extents differ between input and output domains, but now that they are correctly modeled as different by PairwiseRootDomainMap, we may be able to remove it. At least that seems to be the case for torch_gather

[naoyam]

This should be done, but nothing depends on it. Closing for now.