restore generic IndexToScatterGatherOffset specialization (#40349)
Summary:
https://github.com/pytorch/pytorch/issues/39963 erroneously removed template specialization to compute offsets, causing cases relying on this specialization (topk for 4d+ tensors with topk dimension >= 1024/2048 depending on the type) to produce bogus results.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40349
Differential Revision: D22153756
Pulled By: ngimel
fbshipit-source-id: cac04969acb6d7733a7da2c1784df7d30fda1606
